The default units system needs some work but if you set e.g. ‘rem’ for padding via settings, MT will auto-append the ‘rem’ bit after you type e.g. ‘1.5’. It’s a weird system I added way back when MT was young and I was still early in my learning. In practice, most people just leave the default value ‘implicit px’ for all properties and then enter ‘1.5rem’ into the property field by hand when they don’t want to use pixels.
Fortunately, I’m about to update that system so it will be much easier to work with non-pixel units. There will be an option to change the unit in the dropdown menu for each CSS property, along with a slider, plus and minus buttons, and keyboard controls (to increment by using the up and down arrows).
I’m currently working on transform and filter properties, and will release the aforementioned features in the same release most likely, or immediately after.