First, make sure that you are connected to the Internet. The Render[in] License Manager will be displayed the first time that you launch SketchUp after the Render[in] installation or if the evaluation period is over. You can use the plug-in without entering a valid serial number during 30 days. You can display this window by selecting Sketchup menu / Plugins / Render[in] 2 / License...

Enter the serial number (composed of two numbers separated by a dash) that you should receive by email, and click on the Activate button. A message must confirm that the activation is successfull.

NB: To work on another computer, no need to uninstall the plug-in, just deactivate the license number and use it on the other computer. Don't forget to deactivation your license number if you uninstall SketchUp.