(I guess you want to remove the rubber eye cup including the plastic part, right?) You just have to pull it a bit stronger after having pull up the eyecup at his maximum position.
Plastic cylindric part is maintained by 3 small iron ball so please take take to not loose any of them. In order to mount it again after cleaning, sometimes the ball do not maintain all 3 in place. In such case, you can put some soap in the small holes and then place the iron ball (soap will act as temporary glue). Then push the plastic cylindre in, as you have done to remove it.
I hope it helps!
---BF Member since May 2010---