A workshop with details on manually browsing to a game executable in Arcade.

  Most of the time when you log into GameSpy Arcade, it will automatically detect which games you have installed on your computer, and place the appropriate icon under the "My Games" tab on left hand side of your Arcade screen. But what happens when it doesn't, or if it picked the wrong file? You can manually tell Arcade where your game is installed on your computer!  ROCK! If you do not see your game listed under my "My Games" section as you see it on the left here then in Arcade go to the GameSpy menu -> Options

Then when the options box comes up go to the "Games" tab.
Simply find your game listed in "More Games", highlight it, then click on "browse" and find the game's executable file (exe) on your hard drive.  Select it, then Apply and Ok out.  It should then appear under "My Games"! *Hint - if your game IS under "My Games" and you just need to change the exe - look under the "My Games" section in options instead of "More Games". If you know how to look through your hard drive, and where to find your games exe, then GREAT!  If not, around this time you will be staring at your monitor going "ummmmm...." Where is my game installed?
If you have a shortcut to the game on your computer desktop - or even in your Window's Start menu, then you can find the file that launches when you want to play your game.  
Right click on your desktop shortcut and go to properties.   To get to the properties from your start menu right click on what you click on to make the game launch.  e.g.
When you select "properties" you will see a box.
You will notice a field called "Target".  This is where the shortcut of your game points to and it is the path used to launch the  game on your computer.  99% of the time this is the same as what Arcade is going to want to use.  So write your Target down or keep note of it. The actual executable file in this case is "quake3.exe" the rest, C:\Program Files\Quake III Arena\, is where that quake3.exe is sitting on your pc.  

How does this help me on Arcade?

From the Target, we know what the exe is called, and where it is on your hard drive.  So in GameSpy Arcade once again go to the GameSpy menu -> Options -> More Games and highlight the game you want to add.  Then click on "browse".  You will see this:
When you click on the drop down arrow to the right of where it says "Look in:" you will see that drop down menu that appears above.  Now, remember your target?  It was in C: so our first choice would be the (c:) drive!  
From there you would chose Program Files:
Basically you now work your way through those Target folders:  
C:/Program Files/Quake III Arena/ - means go into your C drive then into Program Files then into the Quake III Arena folder.  (Adapt it for the path YOUR game is in).  You should see your exe.
Select it, then hit Open, and then Apply and Ok out and try playing!