Can I use the FDR South to cross Brooklyn Bridge to get to Main St. Brooklyn/? Mapquest chose a nutty route.?

Yes, take the Cadman Plaza West exit off the bridge (right hand side) and bear right going under the bridge and make a left on Adams street and a left on Water 2 streets to Main.

edit: either instruction off the bridge will get you there....

Yes, you can my friend, just take the Brooklyn Bridge exit on the southbound FDR and get on the bridge via the ramp.

When you get to Brooklyn end of the Brooklyn Bridge, stay on the right and take the Cadman Plaza West exit, which will take on the Cadman Plaza West.

Continue north on Cadman Plaza West to Front Street. You'll be passing under the BQE at that point Cadman Plaza West becomes Old Fulton Street.

At Old Fulton Street, turn right onto Front Street and continue east on Front Street to Main Street.

You are now at Main Street.

I hope this information is very helpful.

Good luck