Your weapon or armour has to be charmable. Just hover over the weapon/armour with your mouse to bring up the info. If it says "charm(3)" it means you can add 3 charms (or in your case 2, because you only have enchant at level 2). Not all weapons are charmable.
When you found a right weapon, simply click on it while it's equiped to bring up the enchant window. Drag all the charms you want to this window.
If your charm window doesn't appear, it must be off the screen (but this problem was fixed in the last patch I think). In this case you have to switch to a higher resolution, and drag the window to the center of the screen.
Don't forget that when you place a charm: you can't take it out ever again. So choose very carefully.
Tovi 
