I got this error today while attempting to add a new site to a WSS 3.0 Site collection. I was attempting to add the Knowledge Base application template, but found quickly that the issue was applying to all application templates. You know, all the .wsp files. I've seen a few people asking how to fix this but no one found a solution, what ended up working was actually quite simple. This error comes up when the ApplicationTemplateCore.wsp(ATC) does not install correctly.
When I installed all the .wsp's I in my infinite amounts of laziness created a batch file, which I must admit made installing all 20 of them much easier. I'm sure I'm not the only one who did it that way, and the problem with that is that if you don't watch while its executing you'll miss any problems that come up. I copied the syntax for creating the ATC from the readme they supplied, is there no bounds to my usage of shortcuts? It turns out that it contains a character someplace thats pretending to be something else. The command looks right but when you execute it you'll get a command line error. The rest of the .wsp's will install, and you'll be able to select them, but without the ATC installed you will get the "Feature '75a0fea7-12fe-4cad-a1b2-525fa776c07e' is not installed in this farm, and can not be added to this scope." error. Simply go back, and do this:
stsadm -o addsolution -filename c:\"template directory"\ApplicationTemplateCore.wsp
stsadm -o deploysolution -name ApplicationTemplateCore.wsp -allowgacdeployment -immediate
(that syntax should have all the correct characters)