My neighbor has a 70 Mark Twain with a 70 Mercury outboard. It has a teleflex type steering system. It will not turn. The wheel works, the cables work when detached from the outboard. Do these babies have to be running in order to steer? Is there some kind of "lock" on the steering? Any help appreciated.

No lock.
Pivot is corroded up. Will likely require complete disassembly.

Remove powerhead, lower unit, then unbolt the midsection and pull the lower mount bracket off. Pivot tube will come out the TOP of the pivot bracket.

If it hasn't been frozen too long heat the swivel pin with a torch and put lots of grease in the zerc fitting. If you get it hot enough you can usually break the old ones loose. You can get enough heat with a small propane torch. Make sure the grease comes out the top and bottom and keep moving it back and forth. This only works with the older motors and if its been sitting too long frozen up it will have to be disassembled as mcollistn said.