If you are looking to fill in some low or uneven spots before you bed your electric mat into Firmset--
consider using 'Feather finish' or even vinyl cement patch.
If you need to level the floor check out 'Jiff Set'
All of the above work over new cement----There is a technical help phone number on the bag of stuff that you bought--Call it and ask if it's the right thing for your use.---Mike--