How do you load staples into a Panasonic AS-300N?

Staples newly purchased will always be attached when purchased and that answers your question...wherever the device u are loading has a horizontal channel, there will be a loading window at the end. Take it apart by realeasing the cap or spring at the bottom and sliding the train of staples into the channel inside. Replace cap or spring lever to original position and secure. Stapler is now loaded.

1. Panasonic AS-300NN-A Panasonic Commercial Electric ...

   electricandbatteryofficestaplers.julietshop.tk/panasonic-as-300nn-a-panas...Mar 19, 2015 - Commercial-grade electric stapler with 25-sheet capacity features jam-free operation, auto-reverse, adjustable stapling position and LED status ...

I have one right here. Push the button in the back (above where the power cord enters the machine.) The tray holding the strips of staples should pop out of the front of the machine. If it doesn't, try grasping under the staple tray and gently pull the tray out while pushing the button in back.

The button is a release button, while the tray is spring loaded to pop out..

Try loading with fewer staples to see if that helps and make sure there are no loose staples jamming the mechanism.

there should be a button / lever at the rear press it and it should open so as to insert the staples

Fire the stapler with the load chamber open and down, see if anything falls out. Point it in different positions and fire it, just make sure it cannot fire anything at you.

HERE IS THE LINK TO THE MANUAL
http://downloads.linksysbycisco.com/downloads/WRT300N_ug,0.pdf

Yes the plastic lever triggers the sensor, it may not be going back in place. It is supposed to have a spring like action, so that when you remove the paper it goes back to it's original position. check and see if it gets stuck, or maybe try blowing on the sensor , could be dirty.

it means that your finisher staple motor jammed. try to remove the staple cartridge and anything that might make the staple not to move.