Draw a line on top of the rectangle, give it the line weigh you want, then group it with the rectangle. There's no way I know of to do this with just a rectangle.
VW and Lion are compatible, are you updated to SP5? If so it sounds like your workspace is out of date or corrupted. Try switching to another workspace.
Please post some specs of your computer, processor, ram, video card and version of VW. Use the My Stuff link at the top, Edit Profile and put the info in a signature.
Here is a compatibility list for VW and Windows:
http://kbase.vectorworks.net/questions/875/Vectorworks+Operating+System+Compatibility+List+7-20-2011
Vectorworks 11 isn't shown, but since Vectorworks 12 isn't compatible with Windows 7, I would say VW11 is not compatible also. You also didn't mention whether you have 32 or 64 bit Windows 7.
You may be out of luck trying to run it on Windows 7.
After double clicking on the symbol and entering edit mode, try right clicking on empty space and look at the dialog. It should tell you which mode you're in and allow you to switch to 2D editing.