When you open VBA and it asks you which game to open, click Cancel. This does not close VBA. Go to options of VBA and click Add codebreaker/GameShark codes(they are unable to be used while a game is on this is why you click Cancel). After you enter the codes click Ok. Then go to Open Roms and choose Emerald. To activate the codes I have found you must press a combination of keys similar to no$gba. These keys are Left and Right Option buttons on the keyboard along with the Left control button. I'm not entirely sure how it works, but it's the only solution I've found.
Warning: The codes for Emerald are really buggy and crash the game frequently, by exiting VBA it will remove the codes and your game will be normal.
The only codes I've found to work are the Pokemon modifiers on Supercheats.com
Hope that helps.
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See www.gameshark.com or supercheats.com
turn on your vba+emerald and on the upp of the screen and click cheats then click cheat list and click gameshark type a name and the code
4abcm2fhtmlf1u2c3k4
The code is b505db41 6e39ea4e =)
By Useing Gameshark or other cheat codes.