It's really quite simple, especialy with the primatives props that come with Poser. You simply load the primative into your working window, click on the grouping tool. Click the "New Group" button and create a group called Cylinder and add all the polygons from the side of the cylinder.

Create a second group called ends or two groups called top & Bottom and add the polygons for the ends to these groups.

Then click on the "Spawn Props" button.

Delete the original cylinder and you have left over an open cylinder with either two props called top & bottom or one prop called ends. You can then group them back together in the Heirchy window and the ends can open and close. Whallah. Simple as Sugar! Easy as Pie!