I'm new to using XSI. I've made my first model (the wall-with-a-door example from the Basic Modeling tutorial), and i have no idea how to export it. The tutorial assumes you have XSI Foundation, which i do not. I have 6.0 instead. Can anyone help me export it or teach me how?

if you have the 6.0 foundation look here

but if you have the 6.0 modtool look here

I followed the crosswalk export thing correctly, and saved my object to my computer, but it is in .xsi format. How do i make it so it is usable?

To export a model to the .msh format used by SWBF, you need the FULL version of XSI (either Foundations, Essentials or Advanced, Ver. 4.2- 5.11 recommended). XSI 6.0+ and any version of the modtool (4.2 or 6.0) will NOT work with the Pandemic Addon.

So find someone that has the full version to convert it for you. Once you finds someone, you'll need to workout the details with them.