A generic answer. The unzipped t-shirt file probably contains a figure file (.cr2) and a texture (.jpg or maybe *.tif). Put the texture file anywhere inside your textures directory (which is inside your runtime directory). Put the cr2 file in any subdirectory of the Figures directory. (I have, for example, a "web-clothing" subdirectory that I created). That's it. Now the t-shirt should be available within Poser just like any other conforming clothing.