When I tab a DOS box (cmd.exe) the tabbed window doesn't fill the tab. What can I do to fix this ?

This is because the properties of a DOS box are set to a maximum width and height. These can be changed and set to a higher value in the properties window.

I have purchased WinTabber, but I still get a popup asking me for my activation key

WinTabber works with CNET and Protexis which provide ESD (electronic software distribution) options and copy protection. On some systems the copy-protection software does not work as supposed. If you are experiencing problems you can contact me at wintabber@gmail.com.