Comment 35 for bug 416513

I am seeing the exact same issue with an OfficeJet 6500a Plus e710n-z on Ubuntu 12.10 using HPLIP 3.13.4

I fixed it by running "system-config-printer" from the CLI, deleting all instances of the printer (looks like the HPLIP may have created duplicates) and adding it back.
Then going into the setting of the printer (Right-click, Properties)
Selecting "Installable Options" and checking "Duplexer Installed".
Ok all that, log out and in again.
(This is the exact same as editing the ppd, but might be preferred by those who like a GUI)

Checking in HP shows the duplexer no enabled BUT the main issue of the controls being greyed out is still present.