If you using Unleashed firmware - You don't need this! App comes with every dev build and release version except c build!
Just open Apps -> SubGHz and select POCSAG Pager
In other case
Follow this link:
https://github.com/xMasterX/all-the-plugins/releases/latest
Download all-the-apps-base.zip file
Unpack it, open this folder base_pack_build/artifacts-base/Sub-GHz
Copy pocsag_pager.fap to your flipper into apps folder
Then Open Apps -> SubGHz and select POCSAG Pager