This integration is a natural part of the set up of EstimatorFLEX. Ensure you have this loaded correctly when you edit the "Config.js" file for your propricer server/port and "Key" and publish your tool. Each time you publish and update - these setting will need refreshing so make sure you double check and test.
Once you have published your estimatorFLEX tool - you need to ensure that the connection is working correctly. Users will only be able to access ProPricer from the EstimatorFLEX tool if they have access to ProPricer to begin with, and the setting in "FLEX" allow them access.
Make sure the location that your ProPricer CE is published will allow your EstimatorFLEX tool to "talk" to it.