The integration with Trello lets you import all of your boards from Trello into Forecast. This is a simple migration tool, and will thereby not synchronize new data on the fly.
Step 1: Go to the Trello integration, click here.
Step 2: Click Migrate
Step 3: Login to Trello, and authorize the connection
Migration will start, and your Trello boards should appear as new projects in Forecast in a few minutes.