Staple guns are not created equal. They come under several brand names, including Stanley, Bostitch, Arrow and Craftsman. And as often happens when competing manufacturers develop tools, the accessories that go with them are specific to that tool and usually offered only by that manufacturer. Consequently, Stanley staples won't fit an Arrow stapler, Bostitch staples won't work in a Craftsman stapler... you get the idea.
Besides taking brand-specific staples, a staple gun from a particular manufacturer has its own particular loading procedure. All staple guns have a magazine to hold the staples, but methods for loading the magazine differ. Thankfully, there aren't that many variations, and you can usually figure out how to load a particular tool, such as an Arrow t50 staple gun, by looking at it. If not, there's always the manual. For example, if you can't figure out how to load your Stanley stapler, your best resource is an online version of the Stanley staple gun manual.
Loading a Staple Gun from the Rear
Arrow and Craftsman staple guns use a spring-loaded plunger to keep the staples jammed against the front of the tool. To release the plunger, push in the plunger cap, which is located at the read end of the magazine, and lower it to free the hook from the body of the stapler. Pull the plunger all the way out, drop a row of staples, which must be properly sized for the tool you're using, on the magazine track. Set the plunger back on the track, push it in all the way and lift it to engage the hook and prevent the plunger from springing back.
In the box, Arrow staples are arranged in rows of a hundred or so staples fastened together. It would be great if a single row fits into the empty cartridge, but this is seldom the case. You usually have to break off a small chink before the rest of the row will fit. After shooting 20 or so staples, you can remove the plunger and insert the remainder.
Loading from the Bottom
Loading a staple gun is easier if the gun you have happens to be a Stanley. The Stanley bottom loader also has a spring-loaded plunger that you retract by gripping it from the rear of the magazine and pulling it back. But the plunger doesn't come out. Just pull it out as far as it will go and drop a load of Stanley staples directly into the magazine with the points facing out. When you release the plunger, it will force the staples against the front of the magazine, and a metal flange on the front of the track prevents the staples from falling out.
Loading from the Front
Bostitch staplers, such as the Bostitch T5, load from the front of the magazine. The magazine cover is located right next to the aperture through which the staples exit when you shoot them. Lift the cover to unhook it and tilt it open. Slip the replacement staples on the track that is exposed when you open the cover, then close the cover and the stapler is ready to use. It's an easy procedure, but keep in mind that, if you insert the wrong staples, you can't get them out. So double-check your staples to make sure they are Bostitch staples and that they are the right size for the stapler before you insert them.