from this thread:
http://discourse.mcneel.com/t/alternative-ways-to-initiate-default-popup/4512
The following Mac-only command will display the PopUp tool palette under the cursor:
_ShowToolPalette _ToolPalette=Popup _UnderCursor=YES _Enter
You can change PopUp to any other tool palette name if you wish. You can attach this to a mouse button with Rhinoceros > Preferences > Mouse > Buttons. Use Rhinoceros > Commands and use the Keyboard Shortcuts tab if you want to attach a keyboard shortcut.