Every datasheet that I have looked at mentions the function of pins A, B and C but no mention of D and E. I'm unsure which one is the push button output and which one is the Ground pin. I'm currently looking at this one: http://uk.rs-online.com/web/p/mechanical-rotary-encoders/6234237/
This information is in the datasheet which your (good) supplier has linked on that page.
Figure 1. Encoder pinout. (Click to enlarge.)
Under Electrical Specification on page 194 you can find the encoder pinout. The two pins on the other side are the push-button contacts.
Figure 2. (1) Encoder ABC. (2) Push-button pins.