ajax forbidden

Okay so i saw couple threads like this and quite frankly NONE of them made sense, I installed wordpress YESTERDAY, through FANTASTICO, okay so My problem is when I try to add new categories, I get a FORBIDDEN Message and an ADDITIONAL 404 ERROR, I also try to this also happens again when I try to add a custom field to my posts, can anyone please explain to Me in DETAIL and not just link me to another post because literally I read ALOT of them and I tried CHOMDING, editing the .htaccess and I couldnt find the Mod_Security.conf file that everyone seems to be talking about, please help me out