In the Dimensional Column from the dropdown first define Heat Flux resp. Mass Density then define your units in the Unit column.
The dimensionality "Heat Flux" is correct.
I have made a mystake with the other physical dimension. It was "kg/s^3" and not "kg/m^3".
I had same difficulty to find out from >50 dimensionality types (being new to expressions). So to find out correct type:
Shared above scenario, this will certainly be helpful to someone ahead.
NX gives error while importing expression with unit "kg/s^3", this indicates there doesn't exists dimensionality for this.