/macro dreck loadmap maps/MISSIONS/OUTDOOR_MISSIONS/OUTDOOR_RUINED/OUTDOOR_RUINED_02/OUTDOOR_RUINED_02A.txt
/macro cat "loadcostume atomic catgirl$$rename The Cat"
Something like those. /macro takes 2 parameters. The first is a name that you should make short enough to fit on a button, and everything after that is a slash command without the slash, or a series of slash commands without the slash, surrounded by quotes, and separated by $$.
The macro button should appear in the first empty slot in your power tray. Make sure to open it so you can see the macro.