How Do I Add AR Codes?

[retroben]

Many issues could occur depending on your circumstance.
The codes you have could be for the wrong region.

The codes must be decrypted if they look like "E9FZ-A3QA-YCERP",also make sure the code doesn't still have the identifier line on it upon decrypting it or it will result in immediate crashes.
GCNcrypt is the usual program to use for decrypting AR codes,or you can use the helpful tool on Gamehacking[DOT]org directly within the Gamecube cheats section.

What codes are you trying?
Many certain types of codes can refuse to work due to recompiler getting lazy (like they usually do) with codes that modify execution-code/ASM addresses.

Some size modifiers for anything else other than Link himself will refuse to work and,Cosmo Cortney even mentions so on some of them such as the Seagull Size Modifier code not working on Dolphin.
If you try Interpreter and add then enable that code in particular,it would/should indeed work.