After downloading and installing the base package, you get two executable applications: ERA-RMC and ERA-RMFRP.
In order to install all the book files, you would do something like:
1) Start ERA-RMC
2) Open the About dialog, click to install, choose every RMC book zip you have one by one.
3) Close the application.
4) Start ERA-RMFRP.
5) Open the About dialog, click to install, choose every RMFRP book zip you have one by one.
6) Close the application.
The next time you start ERA-RMC all RMC content will be there.
If, on the other hand, you start ERA-RMFRP, the RMFRP content will be offered.
Each ruleset will keep track of the corresponding characters and adventures you create with them.
Let me know if that was clear, or if you need further assistance or explanations.