After you have activated ExtendSim, the Activation Dialog should not appear again spontaneously. If it does appear, it could indicate that:

The license file (extendsim.lic) has been placed at the wrong location or has been deleted. Verify that the license file is in the ExtendSim folder that contains the user-modifiable files (libraries and example models); by default, that folder is UserName/Documents/ExtendSim10.

There is more than one installation of the same ExtendSim product on your device. For example, you may have downloaded a Trial version to use as the updater for an existing installation. To properly update the existing files, the Trial version must be installed using the exact same folder name and to the same location as the current installation. If that didn't happen, there will be two installations of the same ExtendSim product. The solution is to uninstall the Trial version and reinstall it over the existing installation so that it updates the ExtendSim 10 files.