Greetings
I have a RB3011UiAS and i want to use the last port to provide power to a Ubiquiti Cloud Key
how do i enable this port to power on the device?

Should be auto-on, so just plug it in. But you can see the settings here:

You will need to use an 802.11af power injector. The RB3011 doesn't support 802.11af.

thanks for all responses
So this model does not have the option to power up a POE.....

Which one of the other routers does have this option?

The router can power other MikroTik devices. At this time MikroTik does not have any 801.11af support.