Summary
While the Arcana Scheduler does not have a box that says "Run this program every __ minutes", this sort of schedule can be specified in the Scheduler by selecting multiple minutes in the "Minutes" list.
The "Minutes" list (and all of the lists in the Run as Scheduled section of the Schedule page) allows you to select multiple rows. To do so, hold down the CTRL key while clicking the rows you want.
If, for example, you want to run a program every five minutes, you would select the minutes 00, 05, 10, ..., 55.
Note that if the interval you wish to use is not a factor of 60, you will have to use a shortened interval when you get to the end of the hour. For example if you want to run the program every 7 minutes you would use 00, 07, 14, ..., 56, leaving only a four-minute interval between the :56 execution and the :00 execution.
More Information
The same approach can be used to run a program every x hours or days.