To use your card for building access simply hold the card 3-5 cm away from the middle of the reader. 
If the door is locked, then the LED status will be a constant red.
If you then present a university card with valid access, then the LED on the reader will turn green for a short time and the lock on the door will be released.
If the LED on the reader flashes red, then you are not authorised for entry.
Most external academic doors will be set to automatically unlock within working hours for your convenience.