I had the same problem with an exchange server certificate. But I figured out how to solve the problem.
I used dropbox, - a USB stick will do as well - to get the certificate onto the surface. I saved it to c:\some directory.
Then I doubleclicked/doubletouched/singletouched the certificate in desktop mode and windows opened a popup and asked where I wanted to install the cert. to. As my cert. was a company internal cert. I put it into something with root ... directory or so (in German i.e. Stammzertifikate Verzeichnis), both under user and computer account. Maybe only one place was the right one, and the other not necessary, but anyway it worked.
Does this help?