Hi Dave,
Nice and easy this one.... all you've got to do is remove the chrome coloured pin at the handle end. This then allows the magazine to be removed by pulling it from the front end. Just make sure that you've removed the pusher (Where you feed the staples in) from the back first
Double check your using the correct size staples, there are a lot of different sizes and types out there. I've made that mistake.
The most common reason is that it has jammed on a staple. remove the staples and the rod that holds them in place. See if the stapler will work when empty. Keeping you hand away from the trigger arm Carefully look to see if there if a staple jammed in the release slot. Sometimes you can clear jams with a small, flat blade precision screwdriver.
No, the TRE300 will require a wide crown staple. The staples that will work with the TRE300 are the TRC600 series( TRC604T 1/4" TRC605T 5/16" TRC606T 3/8").
The size & code of staple is usually on the side of the tool , ie 80/ 14 means it takes an 80 series staple & max staple length it can take is 14 mm. Hope this helps