choose:
view > rulers (apple/command 'R' or control 'R')
...right click on the ruler in your document window and select a measurement option...
...to set pixels as the default launch photoshop and choose it in your preferences. Close and relaunch photoshop and it should stick from now on...
...you can also enter measurement values in input fields even if your preferences are different (i.e 10mm, 10px, 10cm, 10in etc)...
Andrew