when I installed omnifocus2 for mac it seems that it looks in the apple agenda, while I am a user of google agenda.
What is the easy way to fix this?
You can pick which calendar is shown in your Forecast view. When you are in Forecast, click on the view menu (the icon looks like an eye) and select the calendar that you want to show in the menu there.
Check out https://support.omnigroup.com/documentation/omnifocus/mac/2.5/en/forecast/ under “Adding your Calendars to OmniFocus” for a more detailed description.
Hope this helps!