Eclipse is looking for eclipse projects, meaning its is searching for eclipse-specific files in the root directory, namely .project and .classpath. You either gave Eclipse the wrong directory (if you are importing a eclipse project) or you actually want to create a new project from existing source(new->java project->create project from existing source).
Goto File > New > Project
Select the type of project, click Next
Uncheck Use default location
Click on Browse to navigate to your source folder, or type in the path to your source
Click Finish