Which version(s) are you using (full version number, please, including the build number)?
I was working on a file, using 6 Freelance demo version. Now I've just installed LSP
Did you de-install the Freelance version before installing LSP?
Aren't the two versions of Trados compatible?
They are (although I don't see why you would need both), provided that you register the version you wish to use when switching (Start - Programs - Trados 6.5 LSP - Register Trados 6.5 LSP).
I suspect you're trying to work with a version that hasn't been properly registered.