(item) is not on the projects build path

(item) is not on the projects build path

I decompiled a jar, imported the files into eclipse, and edited a few key
java files. Whenever i try to export however, it gives me a "JAR creation
failed. See details for additional information.
UHC-2.4.1/META-INF/MANIFEST.MF was replaced by the generated MANIFEST.MF
and is no longer in the JAR. uk/co/eluinhost/UltraHardcore/features/core
[in UHC-2.4.1] is not on its project's build path Unable to get package
fragment root:
UHC-2.4.1/uk/co/eluinhost/UltraHardcore/features/core/GoldenHeads.java
uk/co/eluinhost/UltraHardcore/features/core [in UHC-2.4.1] is not on its
project's build path" error. I have searched far and wide for a solution,
but have nor found one. I have tried refreshing the project etc, and all
the necessary libraries are specified. Please help? Additional
information: If I edit any other class (just add a space then delete it
and then save) that class gets added to the error message as well.