What Does Rated Load of a Tactile Switch Mean?
First, yes you can use the switch with that microcontroller. Its 50mA rating means you can't use it for higher currents, say to drive a motor directly.

Second, if an IO pin on a micro can withstand 6 mA, that means it can supply 6mA when it is an output, for example to drive a LED, or to drive a transistor to control a motor.

If you're connecting a switch to it, you'll also set the pin (in your program) to be an input, and the current it will take is tiny (microamps).

Normally you would either: 

connect the switch from the pin to 0V, and a resistor from the same pin to +v, or
connect the switch from the pin to +V, and a resistor from the same pin to 0v.

The resistor will have a fairly high value, maybe 5k or 10k (or up to 47k). It pulls the pin to the supply rail it is connected to : if +v it pulls the pin high, and you will read '1' in the micro. Press the switch and it will pull the pin to 0V, conducting the current through that resistor (a milliamp or less) to ground. And as the pin is now 0V, you will read '0' from that pin in the micro.