Error: Could not find or load main class in Eclipse

It appears that the project is not part of the classpath.
Check in your Run Configuration->Classpath and if your project is not there, then add it under User Entries.

Eclipse does not support the use of a colon or hyphen in project names. Colons appear to break the way it handles compilation.

The issue can be fixed by not using a colon in the project's name.

As noted in the comments, this may be a fault of Java 6. It is fixed in Java 7.