Same problem here. (Windows 7, 64bit)
In Sonar X2 Studio, TH2 worked fine. Then I upgraded to X2 Producer. Now only TH2-Producer shows up, but not the full version TH2-64. (TH2 still works fine as a stand alone though.)
It's in no sub menu, nor does a vst plugin reset do any good.
I de-installed TH2, re-installed it. Didn't help.
I copied the dll from the sub folder direktly into vstplugins. Didn't help.
I deleted the remains from TH2-Sonar (Studio-Version). Didn't help.
Also deleted the entry from TH2-Producer. Didn't help.
I'm running out of ideas.