^^ You are not alone, others interpreted what happened onscreen just like you did: http://www.trekbbs.com/showthread.php?t=119751&page=49
I assume our interpretation changed over the years because we also saw the Jefferies Tube in use in "The Naked Time" and "Court-Martial" which rather suggested this Jefferies Tube location to be somewhere near the Engineering Control Room in the saucer.
And Scotty's transporter repair in "The Doomsday Machine" via a Jefferies Tube didn't seem to indicate this to be one in the nacelle's pylons which made the scene in "The Ultimate Computer" the most likely "original intention" (?) candidate.
Possibly, after the steps leading up, there might be some kind of one person elevator / trolley platform that would lift a properly suited engineer up to the nacelles (with a few stops on the way, judging by the pylons' windows).
The concept that there is a turbo lift doing the job is entirely conjectural and may originate from the belief that the red vertical lines in the Bridge schematic of the ship refer to turbo lifts.
But these vertical lines could just be the main power / conduit lines, running parallel to these two Jefferies Tubes.
Looks like I will have to add two Jefferies Tubes in my Enterprise
deck plans where the pylons meet the hull.