Im posting this here in case anyone comes across this error and cant figure it out. While validating a map you may get an error saying "Cannot load source/destination schema: Either the file/type does not exist, or if a project dependency exists, the dependent project is not built" .
I got this when creating a map which referenced an external schema (compiled into a DLL) and another schema in a project in the same solution. I had directly tried to 'validate' the map before checking that the local schema project had compiled successfully. Of course, there was no DLL to pick up only a namespace and hence the error (sheepish grin!)