If you've downloaded the map from a server (ingame) then open the console with ~ and type in "map tr_walkway_rc2" (without quotes), or whatever the latest version is. By the time you type map tr_wa the console should be autocompleting the command for you, so just select the option you want.
If you've downloaded the map separately (outside of TF2) then put the .bsp in tf/download/maps and follow the above instructions to launch it (note: I don't know how macbooks work, but that's how you'd install it on a PC; if the mac folder structure is different, I can't help you).