It's under the blue "Customize" button. Under "Move" you can modify existing moves or add new ones and under "Species" you can modify species stats and move pools. Just write the move names into the fields (make sure they're typed correctly though). Save and refresh from the upper right corner above PokeType2.
EDIT: FS/OH Heatran has 16.114 DPS 502.3 TDO, compared to Moltres's 16.127 DPS 434.3 TDO and Entei's 15.418 DPS and 500.7 TDO. The best of both basically and its resistances make the TDO even higher in many matchups.