Is there something equivalent or similar to Jad/JadClipse for Scala? It would be nice to be able to view the source for Lift from within Eclipse via "Open Declaration".
I don't know of any decompiler, but I think what you want to do just attach the source to the jar containing classes.
Just right-click on the lift-jar in the package explorer and choose properties. There you can specify a source location. I believe that an attached source location will override an installed jad-plugin.
If you're using maven and m2eclipse, you can simply right-click the lift-dep. -> Maven -> download sources. That will download the -sources.jar and automatically attach.