You could try to add the path to this unit on the "Compiler Options" dialog, on "Other Unit files"
Another option it to compile the software on the command line. Put all options lazarus puts for your project (look for them on the Compiler Options dialog), and also add -va
The -va option will tell the compiler to give full output of what is going on, so this should tell you the reason why it can´t find the unit. If can be many things, like the .o file being corrupted.