Gmod Texture Problem

Hello My favorite video game in the world is Gmod but the only down thing t gmod is the stupid textures I use a mac and i dont know if that makes any diffrence Whenever i play jailbreak all the time and i always see pink and black checker marks (I HAVE CSS) and i have Half life 2 and half life 2 lost cost ive done every step that people tell me the part where i click on the game controller and click the check marked box (there is a checkmark in all the games boxes that i have) Can someone please help me and tell me how to get rid of these stupid pink and black checker marks

Follow the first tutorial.