Here's a specific problem I ran into when downloading a project from the internet.
Make sure you unzip correctly if it's zipped, you can sometimes get an extra level.
Make sure you place the project in the project file, not directly in workspace.
See if .project and .classpath have been renamed to _project and _classpath. You can't rename them directly so open a text document called .classpath and paste _classpath 's contents in there, saving as all files, not a .txt. _classpath can be opened with notepad.
Import the project from the file workspace. It will look for a folder called projects, your's should be inside it.