in the Opera/program/plugin subdirectory, just make sure that it shows 
the  following files (can be copied from elsewhere, for example from a 
failed  IE installation):

- Squeak.exe (3.7.1 is O.K.)
- NPSqueak.dll (1.0.0.1 is O.K.)
- Squeak3D.dll
- SqueakFFIPrims.dll
- SqueakPlugin.image

It will create Squeak.ini (and/or SqueakDebug.log ;) for you, just 
drop  the .image onto the .exe in the same directory for a test (and 
check F2).

Found the .image at

- http://squeakland.org/installers/SqueakPlugin.image.zip

Stop/start Opera after you copied NPSqueak.dll !

Then in Opera (tools > preferences > advanced > downloads) associate 
file  extension "pr" with mime type "application/x-Squeak-project" and 
select  the NPSqueak plugin.

If the latter didn't help (mime type made some trouble here) I could 
post  the mime type entry for hacking Opera's ini file.

/Klaus