I have been looking through the core rules for DH, and not been able to figure out if PC's are supposed to be able to gain more hit points when they reach the next rank level for there careers. Is this in the errata, have I misread the section(s) pertaining to this, or is a player locked in at what ever their max hp was when they created their character?

Many ranks have the talent Sound Constitution (in some cases multiple times), which adds 1 wound and stacks with itself. This is the primary means of gaining wounds as you rank up.

Sound Constitution: +1 Wound

Each time you rank up, (depending on your career), you can buy a certain amount.

Some classes can buy one at a time. Others can buy multiple per rank.

Dark Heresy characters don't have 'Hit Points'.

They have 'Wounds'.

You don't inherently gain more 'Wounds' as you gain more experience (although you can increase them through buying talents like 'Sound Constitution').