This all depends where u have your sensor if out doors keep out of sun light and a northern exposure is recommended If your temperature is off on your indoor readings consider where both sensors are
You mean time is only coming in AM. The clocks are usually 12-Hour display. So it should automatically turn to PM once you go past 12 hours. I am assuming you have the manual and you are correct it does not show separate setup for AM or PM. You simply keep increasing the hour until it passes 12:00 and it should go to 1pm. Similarly once you go past 00 hours (midnight) it will switch to AM.
Hope this helps.
Manual for reference (won't help you if you already have it)
http://www.acurite.com/media/manuals/02010_02014_02015-instructions.pdf
