You should be able to access all the ideologies, with some specific options within each tree presently disabled. You will be automatically prompted to unlock the pragmatic and malevolence trees if you make the corresponding selections when colonizing planets.
In other words, when colonizing, select pragmatic or malevolence choices as you like, and once you have enough points in those particular ideologies, you will be prompted (with "ideology" in the place of the "turn" button) to select an ideology. Then, simply click "unlock"--though for second tier ideologies and greater, you'll have to select which item from the appropriate tier before you'll be able to unlock.
Also note that presently you must complete each tier within a ideology tree before moving down to the next. You can, however, select from one, two, or all of the trees so long as you've acquired the corresponding ideology points. On large maps, I generally work my way down the trees of each ideology in order to unlock the point-generating planetary improvement (like Missionary Center from the "Empathy" option or Outreach Group from the "Arbitrator" option)--once these are unlocked and the corresponding improvements built, they will generate points toward their respective ideologies.
Hope that helps!
