You really need a manual for this work. The problem may be the cable, may be a problem with the drive, may be a misadjustment, or may be a problem with the shift interrupter switch. The interrupt switch actually interrupts the ignition for an instant when you shift. This relieves pressure on the gears and dogs and allows the drive to go in and out of gear.
Here's the factory service manual for the Gen 11 Alpha drive:
http://www.boatfix.com/merc/Servmanl/14/14cover.pdf
Here's the Gen 1 manual:
http://www.boatfix.com/merc/Servmanl/6/6covr3.pdf
Here's the 4.3 liter engine manual
http://www.boatfix.com/merc/Servmanl/18/18covr2.pdf
The tables of contents are hyperlinked to the chapter files online. Save them all if you want to have the whole manual.
Here's a link with a whole bunch o' Merc manuals and guides
http://www.boatfix.com/merc/mainmenu.pdf