A new menu command was added to the Play menu - Use Classic Accelerators. It acts as a toggle: When ON, the keyboard shortcuts for menu commands used by prior versions of LaserTank (4.x and earlier) are active. When OFF, the new-style shortcuts used by V5.x and later are active. Its value is remembered between program runs.
So... with this turned on you can use F2 for play, F9 for edit, U for undo, etc... most of the shortcuts are implemented. Note that not all the classic shortcuts work on Mac OS X, due to conflicts with the default system-wide keyboard shortcuts on that platform. This can be fixed by going to System Preferences and reassigning the offending keys.