Area of brain that allows us to walk up the stairs without thinking about the actions?

Posterior parts of the frontal lobe, the supplementary motor area (SMA) and the premotor cortex (PMC). A typical landmark on the human brain is the central sulcus, which is a large fissure or valley basically half way through the brain, if you're looking from the side ("sagittal"). The motor cortex is immediately anterior, or more towards the front of the brain, to the central sulcus. The PMC and SMA are anterior to that, and allow us to learn motor actions (PMC), and perform motor tasks without cues present or from memory (SMA).