You would normally receive a clearance to be level 30nm before XXX, for example, or to be level abeam YYY.
In the former case (on my aeroplane at least), you enter a waypoint in the "Legs" page as XXX/-30. That will give you a new waypoint, 30 miles prior to point XXX on your current inbound track. You can then apply an altitude constraint to that point.
In the latter case, YYY can be either on your route or not. Either way, you can enter point YYY into the "Fix" page. From there, select the "abeam" option. This will work out the point on your track which is abeam YYY. This can now be inserted into your route on the "Legs" page as a "place/bearing/distance" from YYY. You can then apply an altitude constraint as above.
Hope that's clear.
