boring standalone problem - can't find Runtime

I'm getting a mysterious error when trying to build a standalone:
Starting Build for Untitled.mxf...
? error: could not find Max/MSP Runtime application
I can reproduce this by opening max 4.6.1 on my Mac Mini
make a small patch
build standalone.
once I press "Build" I get the error in both the Max window and the Collective Editor.
What would cause this? I can't think of anything that has changed in the past few days since I last created a standalone.
I printed the current paths in the Max window, and those look fine.
-- current default paths:
Macintosh HD:/Applications/MaxMSP 4.6/

And, Yes, Max Runtime is in the MaxMSP 4.6 folder.

Help!

Peter.