Bish, The motor should swivel on two bushings. If you cannot swivel the motor by hand (steering cable disconnected), those bushings are likely corroded and frozen. You can try some penetrating oil or heat on the bushings, and see if it will loosen up, or disassemble the motor (remove powerhead and lower unit), and clean 'em up.
If you have a single push/pull steering cable, you will need to unbolt it from the motor and then try to swivel the motor.
Well, if it's between an 87 and 93, you are going to need Mercury service manual #90-13645--2. Merc didn't make a 70hp 3cyl after that, it changed to a 75.
Might try ebay or your local Merc dealer to get one.
The owners manual won't help with the steering.
One question, when you turn the wheel, does the rod on the end of the steering cable that goes thru the aluminum tube move?
You may just have a broken steering cable, or faulty helm. The motor bushings may be fine.
To check your steering, in the diagram below, remove the nut from the link rod where shown, and remove the link rod from the cable end. Then see if the motor will move back and forth.
Also try the steering and see if the rod moves inside the motor tube.
Please, no PM's (Private Messages) regarding boat/engine problems.
That is what the forums are for.
Only forum/moderator issues will be answered in PM's.