I'm no help with naming the tenses (I always get them confused, because we don't always use the same names as English teachers do), but I can tell you that we never use "will" (or, even worse, "may") in the documentation I write. Instead, we'd most likely use one of the following:

Select OK to run the routine. (preferred)

Run the routine by selecting OK.

We tend not to mention the user explicitly, because they're the obvious one to be performing the action. :-)

Ah, but if you're writing an explanation to fellow staff rather than user documentation, that doesn't work as well, unfortunately. "Before this change, when the user selected "OK", the screen turned pink and stuck out its tongue. As of this change, if the user selects "OK", the routine will simply run without editorializing." This isn't "aimed at" users, so the descriptive does have to be used. Alternately, the agent might not even be a user: "Previously, when the midnight compile ran, it sporadically paused to go raid the fridge. With this change, when the records compile, they will have to complete all processes before snacks are permitted." To me, the semi-future tense there is a consequence of the conditional nature of the statement.

Pity. I was secretly hoping that you would have the answer for me!

To my ear, the first option sounds more correct, though I might be tainted by Hebrew grammar, where the future tense is correct in that situation.

Alternatively (not going to imperative), how about "When" instead of "If":

When the user presses OK, the routine runs.

That sounds more descriptive to me.

Note -- I just found the following summary of English tenses