ERA was recently updated, so the first thing you need to do is check that you have the latest version of the base product and the creatures package.
The steps for installing the packages purchased at DriveTruRPG are shown in this tutorial video: https://www.youtube.com/watch?v=OP1_H0zVZuk
Let us know if that answers the question, or if there is anything else we can help you with.
Thanks for your interest in ERA!