I do believe it has effect, i fired a marksman rifle at 100 meters that has a zeoring of 300 meters, his head was in the dot, thus the bullet hit just above his head.

correct but i can't seen to change it

Not all weapons allow to adjust zeroing on the fly

The siper rifle MXM 7.62mm with RCO? If it's not in the I would be disappointed I really liked the possibility in ARMA 2 to change the zeroing om the snipers and scopes.

Not sure if it's really weapon or scopes related.
Try with the other zoom optic (not the RCO, there's a 2nd one)

I've tried both snipers, with scopes but i can't seem to find any difference when taking the change the zeroing.

I even followed the field manual (were there is a explanation) but still no effect

Zeroing WORKs 100%
BUT not for all weapons.. just wait new content and changes

Zeroing does work, at least on the weapons I tried it on. I didn't try zeroing on the "sniper" rifles, because they seemed to have Bullet Drop Compensation reticles, so there is no need to zero them (also those scopes would not have zeroing capabilities).

Correct me if I'm wrong but the RCO/ACOG isn't a sniper scope. It only has 4x magnification and It has marks for correcting your aim in the reticle. Therefore no need to zero.

The 2 4x scopes in the game right now don't have zeroing. they are ACOGs and not actual sniper scopes so that may be your problem.

See #0003540.