First off you need to have the appropriate OS for your device loaded on your PC. Once loaded, find the blackberry.alx file. It will generally be found here:
C:/Program Files/Common Files/Research in Motion/Shared/Loader Files/(file name/number you just downloaded and installed)
**The last folder is the OS you have loaded on your PC**
Once found, open the file using WordPad, and look for something similiar to this example (note, this is example only)
Note that the only thing removed was _vendorID="102"
You can do this for all the theme files and hence have all vendor themes unlocked for your use. Then just Save the file. Close all the open files, then open Desktop Manager, connect your device, and it should automatically ask if you want to upgrade. Just click yes and you should be good to go. If it doesn't, you can open the App Loader and do it manually.
Now if you are using an OS other than one provided from your carrier, you'll also need to delete the Vendor.XML file. It can be found in the App Loader folder located in the same main folder as the files/folder above.
Give this a shot, if you get lost, come back here and let me know.