Compiling into a .jar on Eclipse

I'm using Eclipse, and I need to compile my project into a .jar file, but every time I try to run the exported file, I get:

Failed to load Main-Class manifest attribute

I've googled it, and from what I've discerned from the results, nothing in the manifest is point to the class with my main() in it. Only problem is, I don't know how to fix it. I can unzip my .jar, but there's nothing in the MANIFEST.mf file, and even if there was, I wouldn't know how to put it back together.

Thanks in advance.

When you export your project in a JAR file, one step of the export wizard is called "JAR Manifest Specification". In this page, you can select your own MANIFEST file. So make one, specify which class is the main class, and JAR everything using that file.