EVM is meant for ESF v1.1, not v1.2. You'd really have to be more specific on which ESF version you are using, if you are using a valid copy of Half-Life with Steam, which EVM you are using, and the list goes on.
As I mentioned before, EVM does not work with ESF v1.2.x, it has been made for ESF v1.x. If you wish to play with an addon for ESF v1.2.3, then I highly suggest you to use ECX (Evolution Class Extension) Which can be downloaded from:
EVM 1.X was for esf 1.1
EVM 2.X was for esf 1.2.1 or 1.2.2 ( dont remember )
ECX was for esf 1.2.2 and 1.2.3
and absorb works like this:
you have to be very close to the enemy ( < 300 units ), then aim at him and activate special2. you should start glowing and through that you should get hp or pl ( dont remember ) if you press special2 again during this time, then the absorbation will be stopped