the sma and includes aren't encrypted. -.- infact under GPL all these scripts have to come with sources. So if ur refering to the amxx file when u open it, thats not encryption, its just compiled. Just open the sma's and the inl, .inc files. I doubt u'll b able to mod it if you don't even bother to search for such a simple answer as opening a sma file.
Well I didnt know this about AMXX files. I havent really coded under HL engine that much so didnt know about this. more of my exepernice is in the Darkstar aka tribes 1 engine.