License keys are valid for all versions; we have updated our licensing mechanism in 2008, but any license given out since 2008 should work in any version of the software from after 2008.
If pasting won't work at all, here's another workaround:
- Make sure that Stereo Tool and the VST host application are closed
- Open the fiie C:\Users\username/vst_stereo_tool.ini in a text editor
- Search for a line "Registration key=", and paste your key (including the < and >, no extra spaces) after it
- Save the file
If you start Stereo Tool again now it should be licensed.