I was getting the same error, with Offline and online. It's running now, here is what I did:
used the Download-All-SP2010_PreReqs.ps1 - Result: PrerequisiteInstallerFiles (with files contained within)
moved PrerequisiteInstallerFiles to folder SP2010
copied PrerequisiteInstaller.exe to SP2010 Folder. <-This may be what it is looking for
It appears to be working...at least with Offline set to false.