Foreign keys and queries (no replies)

Hi
I am coming back to SQL after a twenty year break so have enough knowledge to confuse myself without being any help.

I have set up a model in workbench and successfully exported this to a new connection I created. However this was only after I removed the foreign keys I had created in a table. I have user table, an activity table and a user activity table with the user activity table containing foreign keys from the other two tables.

However I had to remove those foreign keys before I could forward engineer. Why is that?

Now I have the three tables in the DB and have created some records in each I want to show the activities that each user has signed up for. So it is some sort of union/join statement I think, but how do I build this through drag and drop of the relevant columns from the different tables?

thanks for your help
Nick