Download the latest version of FM MailBridge.
It is usually provided as a ZIP archive.
Unzip the file – inside, you’ll find a file with the .fmaddon extension.
Launch FileMaker Pro.
Open any FileMaker file (e.g. a test file where you want to try out the add-on).
Switch to Layout Mode:
Menu → View > Layout Mode
Open the Add-Ons palette:
Menu → Window > Add-Ons
(in some versions, also accessible via the right sidebar)
Click the ”+” icon or “Install Add-On”.
Select the .fmaddon file of FM MailBridge.
FileMaker will now import all required objects (layouts, scripts, tables, etc.) and prepare them as an add-on.