Highlight (select) the license key right in your email. Then, to insert it in the key-entry dialog in your TwistedBrush program, use the Windows copy-and-paste functionality--Ctrl+c to copy and Ctrl+v to paste. Make sure you are not selecting any additional symbols or blank space. 

For TwistedBrush Pro Studio versions 1-22, make sure you enter your first and last name exactly as they were listed in your purchase order. They must match for the license key to work.

Also see the License Key topic in the User Guide.