I had this problem with a Java project that I imported from the file system (under Eclipse Helios). Here's a hint: the src code didn't seem to be compiled at all, as no "bin" directory showed up.
I had to create a Java project from scratch (using the wizard), then compare the .project
files of the non-working and working projects.
The project giving "Editor does not contain a main type" had this as the "buildSpec" in the .project file:
<buildSpec>
</buildSpec>
But the working project had this as the "buildSpec":
<buildSpec>
<buildCommand>
<name>org.eclipse.jdt.core.javabuilder</name>
<arguments>
</arguments>
</buildCommand>
</buildSpec>
I copied this in, and the imported project worked.
I know my answer is for Java, but the same might be the issue for your Scala project.