Step 1: Open HMT, and go to File>>Open, then browse to where your maps are (usually at C:\Program Files\Microsoft Games\Halo\MAPS\) and open a map.
Step 2: Scroll down until you see a line (called a tag) that says [weap] weapon [20 items]. This tag has all the weapons inside it. Click on it to expand it.
Step 3: Look through the sub-items that pop up, and find the weapon that you want to use (I am using the pistol in this tutorial).
Step 4: When you click on it, it should open a panel on the right side, look near the bottom of it, and you should see the word "Projectile:" with an empty drop-down box next to it. Click the box, the only option should be "0". Click that and numbers should fill in the blanks beneath it
Step 5: Final step. Look for the box called "Ammo per Shot:" and change the number beside it (it's 1 by default) to 0. This makes your ammo decrease by 0 everytime you shoot, instead of 1.
Once you have done this, simply go to File>>Close before trying to play that map on halo. Congratulations, you have made an Infinate Ammo weapon!