To set your Status Message first go to your Profile page. After that tap on the little field that displays your current Status message.

Then  type in the Status Message you want and tap the "Set" button

Please be aware that Statuses are considered Public, so you can be Silenced if your Status message violates PIMD's Terms of Use. Use your Status message to tell others more about yourself, offer Trades, welcome players to your Club, or anything else you would like to broadcast from your Profile page.