Where is the OK button for LaserJet MFP M28w (for manual duplex printing)

[Kat270]

The printer will not print the second half of the duplex printing -

after pressing print the first side of the document prints

then nothing happens.

In the app it says:

Status: Manual duplex.
Load tray 1.
Press [OK] to continue.  

where is the OK button?

[Valsimot]

There is no OK button on the printer, open and close toner access door then the printer should start printing 2nd side.

[AlS77]

Here's a link to the User's manual:

http://h10032.www1.hp.com/ctg/Manual/c05847830

See page 4:

In the manual it states:

Start Copy button

Use this button to start a copy job. Or, in manual duplex mode, use this button to print the second side of the page.

This should be what you're looking for.