how does radha heir to keld work

[Footprint]

how does radha, heir to keld work exactly

what if i dont have mountains when attacking

[Arix]

You add two red mana to your mana pool - this is basically the same thing as tapping two Mountains normally.

Note that you have to spend the mana during the declare attackers step, or it disappears.

[DatAsh]

I think you're backwards on the understanding of her ability here. When you attack with her you don't need to add two red mana, SHE produces two red mana.