Unfortunately, Mame support on the Mac is very weak and requires the use of a complicated command-line application. There were some cool apps for Mame on the Mac like MameOSX™ and MacMAME™, but they are no longer supported on current versions of OS X, so regular folks find the command-line alternative complex and difficult to install/use.
The easiest, stress-free way to run MAME games on your Mac would be to use virtualization or a separate partion for Linux and run an install of direct-boot to MAME called AdvanceMAME.
Currently there is a known bug with SDLMAME which requires the X-Arcade to only be connected AFTER SDLMAME starts, and this process must be annoyingly repeated any time you change a rom. (Unplug X-Arcade, Close SDLMAME, Re-Open, Then Connect X-Arcade). We are still trying to determine the cause of this problem, but in the meantime you must be patient. AdvanceMAME runs on Mac using an older version of MAME.
2) Install SDLMAME (direct link to download) for your Mac.
3) Make sure to follow the instructions, directions and troubleshooting. Mame on Mac Technical Support (When you run into a problem you can search the SDLMAME forums or post a question.
To help you out, keep in mind that if your X-Arcade is connected to your Mac, you can open up Textedit or any application to let you see what you are typing on the screen like Pages, Word or the included Mail App. The X-Arcade is recognized as a USB Keyboard, so to verify it's working, you'll be able to press buttons/move joysticks and see the responses on the screen. When that happens, you'll know X-Arcade is fine, you just need to change the settings in the app or game to use those keys.
To use with a specific game or application: you will need to make sure the game/app allows keyboard inputs to be programmed in the game settings OR you can connect an old-school PS-2 Keyboad to your X-Arcade and re-program the keys to match the keys assigned in the game/app.
Step 2: Connect Your X-Arcade™ to USB Port
If you are using the Tankstick, connect both USB connectors to two USB ports. One is for the Dual Joystick (keyboard) setup, one is for the Trackball (mouse).
Step 3: MAME™ Operation:
By default, MAME™ needs a few minor adjustments for your X-Arcade™. The default keyboard commands are identical to the X-Arcade™ with the exception of the left Player 1 Joystick and coin-insert feature.
After you start a game in MAME™ hit the TAB key to enter the configuration menu. You can choose to change the "Default Button Layout (General)" to use the configuration for all of it's games, or "Default Button Layout (This Game)" to change just the configuration for the game you are currently playing. We suggest "GENERAL"
Setting Up Player 1:
Using your keyboard, Select Player 1 Controls. Select Player 1 Up Press Enter on keyboard Move Your Player 1 Joystick Up Repeat these steps for Player 1 Down, Left and Right Press ESC To Return To Main Screen
Setting Up Player 1 For Accepting Coins
Scroll Down On Screen To Player 1 Select Press Enter Press The Left Side Pinball Button (Or Press 3 On Keyboard) Repeat For Player 2 Select under Player 2 Controls Press ESC To Return To Main Screen
*NOTE: If after this procedure Macmame still does NOT save the settings. You must delete the config file and start over
Open Finder to Documents/MacMAME User Data/Config
Setup Player 1 Again
Step 4: Playing MAME™
After you start up a game, use the left pinball flipper button on the left side of the X-Arcade™ to insert coins, and press the white start button to play.
You can also skip the screens which ask you to type "OK" by moving the joystick left then right (left for "O" and right for "K").
You can exit a game by holding the player 2 start button and pressing the right pinball button simultaneously.
You can change the controls for any game by simply pushing the "Tab" key while a game is already running to access the controls menu. The Input (general) option allows controller settings to be applied to all games. Input (this game) overrides the general input settings and adds custom controller settings for just the game being used.
If you aren't able to certain Roms working on either client, you must make sure to use Rom Versions matching the version of MAME running.
Not Recognized? Make Sure Your X-Arcade™ Is In Mode 1
Most problems are accounted for by checking your in the right mode. While someone like Woz would love it, it's quite an annoyance to us Mac™ users just trying to relive some classic arcade games. Mode 1 is the default mode and if you're switch is moved closest to the serial port, it's fine.
If it wasn't in this spot, move it there, then push the button on the back of the X-Arcade™ twice (the light on the X-Arcade™ should turn off from the first push, and back on with the second) and you will load Mode 1
The MAME™ software includes a "Help" menu that can help you (press F1 on your keyboard to access). Alternatively, here are some more very helpful sites that have more in-depth MAME™ instructions, troubleshooting help, and information: