All MIB files you attempt to compile in MIB Tools result in the error, "The selected file does not appear to be a valid MIB..."
This can happen, of course, if the MIB files are invalid. For example, they could be missing a valid DEFINITIONS statement. It can also happen if the disk is full where OneClick is running.
Please reference the "Managing MIBs and Traps With MIB Tools" section of the documentation for more information.