There is only a visitor's terrace on Terminal 2 in Frankfurt anymore and no, transit passenger don't have access to it.
It is accessible from the check in area outside of the passport control (like it is with all other airports I guess as they want to make it available for the general public).
It is not the best place to take pictures of LH
Aircraft, as they are on Terminal 1. LH
widebodies will only occasionally taxi by and you can see some narrowbodies on the apron.
The place in itself is not bad but you will shoot against the sund during daytime and it closes at 18:00, before the light gets better.
After all it is only open from April to October (and sometimes in November).
The information mentioned above seems to be very old when the terrace on terminal 1 was still open, it closed in the early 2000's.
In the transit area you could only see airplanes through some windows.
For better views either go to Terminal 2 or leave the Terminal direction runways.