If you see the dll file, but it is not somehow available for Visual Studio to use, perhaps its state isn't correct:
In command-line, go to the parent folder of that file, and do a:
cleartool ls
cleartool descr -l yourFile.dll
Maybe it isn't readeable, because it is taken by another process.
The other reason a file isn't readable is mentioned in "About the pathname limitation on Windows and its affect on ClearCase" and this technote:
There is a limit of 255 characters for any label, branch, hyperlink, or attribute name.
Make sure no pathname is too long in your project.