Make sure that WoW (or the PTR if that's what you have hooked up to WMV) is fully updated. Go to the Options tab in WMV and click reset, which will open the paths you have available to hook up your version of WoW. Your version of WoW must be MoP or PTR versions as well in order to access the MoP items as far as I know.
You can also try clicking the File tab and selecting Discovery Item.
If none of that works I suggest trying the WMV forums to ask your question. The develops answer there and they know way more about it. http://wowmodelviewer.org/forum/