I installed the DX11 and the High Resolution texture pack from mycrisis. I have an Nvidia GTS 250 with 1gb of GDDR5 RAM. The graphics card is Direct X 11 compatible and it fits the requrements for the DX11 and the High Res Texture pack. I don't know why I can't enable the option, when I tried to install I got the error "On Startup, Line 91: attempt to index global 'tbVersionInfo' (a nil value)" but by reading around the forums everyone says that this doesn't affect anything. I need help!

I think you mean the GTS 450 because the GTS 250 is only DX10 and has GDDR3 ram.
so make sure what GPU you have.

have you updated the game to DX9?
also refer to the FAQ at the top of forum