1. Run WinMPQ
2. Go File > New and choose a place to save your mod's MPQ. This will become the file you'll load with MPQDraft and may even distribute to other people as your mod, so make sure it has a suitable name for your mod.
3. Now WinMPQ has created your MPQ and is waiting for you to put files in it. Drag and drop your units.dat file into WinMPQ.
4. It will ask you for a prefix. Now remember that for StarCraft to be fooled into reading your arr\units.dat file and not the original arr\units.dat file, you need to give your arr\units.dat the correct name. Type in "arr\" into the box, and this will add "arr\" to the front of your filename "units.dat" which results in your file being added in under its correct name, "arr\units.dat". All of those filenames should be entered without the quotes.
That's all there is to it, now you have your mod in an MPQ. As your mods get larger and more complicated, you'll find you edit many, many files, not just arr\units.dat, but maybe arr\weapons.dat too, other dat files and possibly hundreds of different wav files and graphic files. However, for all of these files, the end result is, all the files need end up in the same MPQ. Think of the MPQ like a ZIP archive, except it stores files that StarCraft can read. If you're going to make a mod, you need to "zip" or in our case "mpq" your files together for StarCraft to read them in a format it understands.
Once you have got your MPQ setup, you can run it just like any other mod. Here's a tutorial I wrote for that:
http://www.starcraft.org/customs/CustomsTu...arCraft+Customs although I'm sure there are many other tutorials on the topic online.
Edit: I just noticed you said you're using weapons.dat. Now, everything in these instructions will be the same, except weapons.dat must go into the MPQ as arr\weapons.dat (the name has to be completely exact) so wherever it says "units.dat" up there replace that with "weapons.dat". The same sort of concept can be applied to all the files which you will soon find yourself adding to MPQs, they all have to go into an MPQ, with a correct name, and you can do that with WinMPQ.