I think you are talking about SoulShots, but i am unsure. SoulShots are magically enhanced things that when put on a weapon makes all hits against an opponent do 2 times the damage (a regular hit for 7 would with SoulShots do 14).
All weapons have an ammount of SoulShots they use when attacking something (for example most weapons at the start use 1 SoulShot and later on they use 2), and to be able to use the SoulShots you need to open your inventory, click and drag the icon of the SoulShots onto your bar where you have your skills(make sure its not locked, right hand corner of the bar is a lock, make sure its open). When the icon is there on the bar, right click on it and you will automatically use SoulShots every time you hit something.
If you want to save your SoulShots right click again and the icon on the bar will no longer shine and you will stop using SoulShots.
Remember that each SoulShot has a grade, and has to be used with the same grade weapon (if you have a No Grade weapon you need No Grade SoulShots, and if you have a D Grade Weapon you Need D Grade soulshots). Only No Grade SoulShots are sold by stores, all other SoulShots of higher grade are made by players (dwarfs).