If the files are all there, then is seems that Photoshop has just forgotton how to get there.
There is a file called
PluginCache.Paint Shop Pro that seems to hold this information and which you will find here..
...Documents and Settings\
YourNameHere\Application Data\Adobe\Photoshop\7.0\Adobe Photoshop 7.0 Settings
(OK, this is for 7.0 obviously, just substitute CS as appropriate)
You can try this.
1) Close CS;
2) Change the name of
PluginCache.Paint Shop Pro to
xPluginCache.Paint Shop Pro;
(I wouldn't delete it yet, just change the name)
3) Start CS again; It will take considerably longer now because CS has to localize all the plug-ins from scratch (that's why there was a cache file

)
4) There is now a brand-new PluginCache.Paint Shop Pro file.
With a bit o' luck the problem has solved itself.
If not then
Rô