Important: It's an early BETA that's intended for testing purposes only. Currently, the version is available for Windows only, MacOS one is coming soon. Please make sure you have BACKUP copies of all your projects before installing this version.
Usually, your projects are stored here:
for PC: C:\Users\YourUserName\AppData\Local\Mobirise.com\Mobirise\projects\
for Mac: ~/Library/Application Support/Mobirise.com/Mobirise/projects/