What is considered the adventure screen? how do you enter a cheat code?"

The adventure screen is the main map screen,.

(Not the town screen where you build structures and buy troops, and not the hero screen where you manipulate troops and artifacts)

Not knowing which game you are inquiring after, cheat codes are usually entered on the number keys. If the keypad doesn't respond, try the number keys under the function keys.

You need to click TAB and you will be writing on that tiny 'ribbon' above the resources (ENTER to and apply code). You can probably find specific codes on internet.

PS Numbers were in homm2 - in homm3 it was 'nwcSOMETHING' - PADME, AMIDALA, PODRACER itp, itd