What you need to do is add a new material to a file called EnumToolMaterial.java. At the top of the file, you'll see some things like GOLD, EMERALD(Diamond), WOOD, etc... they're constructed like this.
WOOD("WOOD", 0, 0, 59, 2.0F, 0)
The first number needs to be one more than the number of the material before it.
The second number doesn't matter.
The third number is the number of uses - wood it's 59, but diamond is 1561.
The fourth number is it's efficiency - wood is the least, but gold, the most effective tool, is 12.0F. And the last number is the one that it causes versus an entity - mob. So, this is the one you need to change. Basically, what you want to be doing is something like this:
WOODDAGGER("WOODDAGGER", 5, 0, 59, 2.0F, *The number you want the damage to be - diamond is 3*);