oh, then do what he said(above me) but set the workspace to the src folder(the last src folder) inside the src foldr that is in the mcp folder(the folder where all the java files are in)
No, not the src folder. The eclipse folder inside the main MCP folder.
If that doesn't work, backup your source and the ENTIRE MCP folder. Unzip a fresh copy of MCP, add the jar (and ModLoader, etc), run updatemcp and decompile. Then launch Eclipse and choose Switch Workspace > Other from the file menu. Browse to the main MCP folder that you just created, and select the eclipse folder. Once you select this workspace, Eclipse will always open it. If not, there might be some turned-off setting to remember the last workspace.