...an ardent who attains 5th level can learn any power from one of her mantles that costs 5 power points or less to manifest; she cannot learn a power from a mantle that costs more than 5 power points to manifest until she attains a level capable of manifesting a power with that cost.
Emphasis mine. A Fighter 4/Ardent 2 with Practiced Manifester is at a level capable of manifesting a power that costs 6pp.
thank you everyone for the various readings, this is what i was looking for. i think i better understand where the interpretation is coming from.
however, i do not think it is correct to assume manifester level, when your emphasized phrase seems to be pointing at class level. for every other psionic class, it is not manifester level which determines powers selected/learned at a given level, but rather class level; thus the default interpretation should be class level in this case, not character level as has been suggested... which suggestion i would happen to agree with if this were not a spell/power selection/learning by level example.
furthermore, "attaining a level" is very close with other class level selection, ie: leveling up via xp, references and phraseology. i cannot think of any other single instance, be it caster level or manifester level, where a word like "attaining" is used in the reference... unless i'm just wrong, that is what the designers meant, and ardent is the first such reference.
but i have another thought as further circumstantial support. whenever a new mechanic is introduced, it is usually accompanied by large text blocks explaining it... re: erudite unique pp/d, skill tricks, incarnum, shadow/binding/truename, and so on. if the designers had intended to introduce a new and unique mechanic that represents a departure from every other psionic class and prestige in the game by allowing powers known to be picked by manifester level rather than class level, that there would at least be one explicit mention somewhere.
also, in conversations i've had with the designers, they were required to stick to core and the book they were working on as much as possible, keeping "required to play" books other than core to a minimum. anyone happen to know when the practiced manifester and practiced caster feats were published in relationship to comp psi? if that feat first appeared in comp psi, i would take it as indication that manifester level based power selection might have been the intention.
i think my first and second points are pretty solid.
the third and fourth points are a pair of shaky limbs perhaps, given the many famous editing goofs, but i feel worth mentioning.