Question: My partner and I are working on a project using TFS. For some reason when he adds files to the project then checks them in, I don't get them when doing an get latest version. The same thing happens when I check in files, he doesn't see them. We can both go in to source control and see the files are there, and even get latest version, but they are never added to the solution explorer. Any ideas what is happening?

Answer: 
If he, or you, has checked in the files, but not checked in the project file, then TFS will not download them if you execute a "get latest" by right clicking on the project or solution in Solution Explorer. 

You WILL however get the files downloaded correctly if you right click and "get latest" on the folder in the source control window, however, they won't get loaded into your solution, because you haven't told TFS that they belong there.
Can you verify that the .csproj (or .vbproj, etc. depending on language) has been updated to actually include the new file(s)? Is the project file also being committed along with the new files?