Generalists choose a discipline at each level they gain access to a new level of powers. 

As an Example

A 10 level generalist psion:

At 1st level you gain access to first level powers. You choose the discipline Clairsentience, you now can access all 1st level Clairsentience Discipline powers, they do not count against your total powers known. 

At 3rd level you gain access to second level powers. You choose the discipline Metacreativity, you now can access all 2nd level Metacreativity  Discipline  powers, they do not count against your total powers known. 

At 7th level you gain access to third level powers. You choose the discipline Psychokinesis, you now can access all 3rd level Psychokinesis  Discipline powers, they do not count against your total powers known. 

At 9th level you gain access to fourth level powers. You choose the discipline Psychometabolism, you now can access all 4th level Psychometabolism  Discipline powers, they do not count against your total powers known. 

Dual Disciple / Secondary Discipline 

When the dual disciple chooses his discipline, he also selects his secondary discipline. The dual disciple gains the discipline  powers known from this secondary discipline power list as well as from his normal power lists. However, the dual disciple gains discipline powers one power level lower then his primary discipline when manifesting a power of any discipline other than one of those he has chosen. The dual disciple gains his primary discipline abilities at 2nd, 8th, 14th, and 20th levels as normal. This ability replaces the class skills normally gained when selecting a discipline.

Secondary Discipline Ability: At 10th level, the dual disciple gains the 2nd level discipline ability of his secondary discipline. This replaces the bonus feat normally gained by a 10th level psion.

Secondary Discipline Ability: At 15th level, the dual disciple gains the 8th level discipline ability of his secondary discipline. This replaces the bonus feat normally gained by a 15th level psion.