This one is unlocked by maxing out the Technomancy skill tree (unlock AND upgrade every skill). This will require a total of 24 points, so the earliest this one can be unlocked is level 25 if you're dedicated.
With a full, dedicated playthrough doing all sidequests and most of the combat encounters, you should end up around level 35/36, so it's not possible to completely unlock more than one skill tree simultanously.
If you're planning to unlock more than one skill tree in the same playthrough i suggest you're investing 4-8 points into the tree you're most comfortable with for combat at the beginning of the game and then start to save up points until you've got 24 stacked up.
Then save the game and unlock the first skill tree, rinse and repeat with the next one.
Edit: I've updated the numbers for levels and skill points according to what mortilect and StevesTurn said in their comments. Thanks for the correction, seems I made a mistake there.