Yep - just sounds like a cable adjustment to me as well. Need to set the shifter to the neutral position, pull the cover off the outdrive and check to see the linkages are also in the neutral position. Back off the cable nut at the outdrive end and just shift the linkages there by hand until in neutral then tighten the lock nuts up.
Graham also has a good idea and that is check that the cable outer is secured to the outdrive with the little slidey horseshoe type bracket and bolt. If that's not installed right, then your shifting will be all up the creek and you'll get either all forward or all reverse but nothing else. You can see this in the following pic. Maybe check this first!

The way the cable connects on the back of the shifter is also something to check - they are a bit fiddly to lock on (I found).