To use the heater on a typical electric fireplace:
Some of the controls on your electric fireplace will operate the heater, while the others will operate the flame functionality.
Many electric fireplaces come with high and low heat settings, and may have separate switches for each.
Ensure that your fireplace is turned on at the wall and on the appliance itself. The main power switch on my electric fireplace must be turned on in order to use the heater, but there are separate switches to control the heater.
On my electric fireplace, the controls highlighted in red below are used to operate the heater.