In my limited experience with BizTalk it seems that BizTalk projects and Visual Studio version are somewhat locked to each other. For example, you can\'t open a BizTalk 2010
I had a related problem today. I tried to use VS2013 Ultimate to open a BizTalk 2013r2 solution - which should work fine. I received the dreaded Unsupported message. It turned out that the problem was caused by the fact "Developer Tools and SDK" was left unchecked during the BizTalk installation.
To resolve I mounted the BizTalk Iso and ran the setup.exe. I selected Modify Existing Installation then checked the box for "Developer Tools and SDK"