I take it you're installing the lower unit? There is a hole in the Stbd side of the exhaust housing where the cable comes up into the interior of the motor. As I recall, you'll need to remove the "horse collar" trim piece that's underneath the lower cowling (the one that covers the powerhead mounting bolts).
There are a couple of screws/bolts at the back side of that collar, then at the front there should be (2) screws (or bolts with small heads) on each side that hold the main collar to the small front piece.
Once you have that off, you'll get a better view of the hole where the electric shift cable goes.
Best way to get the cable fished up into the mid-section and thru the hole is to drop a cord, bailing wire, or electric light cord down the hole & out the bottom. Lash up the cord/wire/etc to the shift cable wires and put a tight wrap of electrical tape around.
Don't use too much, or it won't go thru the hole! What works for me is to take a chunk of 2-wire lighting cord (aka "zip" cord), strip the 2 wires back, and wrap each bare wire around one of the bayonet connectors on the shift cable. This makes a strong union of "fish" cord and cable, and isn't real bulky when wrapped with tape.
You'll note a bunch of "bumps" on the shift cable, and a couple of places that are bare; one, close to the top of the cable, and one, farther down, dividing the "bumps" in two.
The farthest-up "notch" in the cable is for a longshaft motor, and you'd only pull the cable up far enough for the notch to seat in the hole in the mid-section.
The 2nd notch is for a shortshaft motor; pull the cable thru, past the first set of lumps 'n bumps, and seat the cable in the 2nd "notch". If you don't pull the cable all the way up, there will be slack left inside the mid-section, and it's possible it'll get damaged by the driveshaft rotation.
Be sure to grease-up the cable with marine grease before feeding it thru. And by all means, scrape out the mid-section cable hole to make sure there's no salt or other deposits/crud in it.
It's hard enough to get that cable thru when the hole is free and clear; virtually impossible if the hole is gunked-up.
Be Really Really careful with that cable; it's close to $200 if you can even find one, and they're not available from the mfr anymore.
Well, that should just about do it, Have Fun!.........ed