Data, like his "brother" Lore, was created by engineer, scientist, and inventor Dr. Noonien Soong on the planet Omicron Theta, which was stripped by the so-called Crystalline Entity — see "[Datalore][1]".

If you watch this episode and other episodes of *TNG* dealing with Data's back story, you will see that the colony on Omicron Theta was not especially technically advanced.  Given that Soong's primary goal was to create a fully functional positronic neural net (Data's brain) inside a humanoid shell, and given the general lack of technology elsewhere in the colony, I would say that it was not high on Dr. Soong's priority list to give him the ability to access computers and other electronic devices by non-humanoid means.

One could ask, why wouldn't Starfleet or Geordi later augment Data to have that ability, given how useful it might be?  It was emphasized many times in *TNG* that most members of the Enterprise D crew consider Data to be an autonomous life form, and so they likely would frown upon such suggestions as disrespectful.  (Earning this respect was not always easy: Dr. Pulaski, case in point.)

But actually, Data's brain has been directly "plugged" into the Enterprise computer and other computer systems many times in *TNG*.  So it is not true actually that Data always has to "use" a computer manually in Star Trek.  (See the nice screenshot in Einer's answer above.)

Furthermore, by interfacing with Locutus in "[The Best of Both Worlds (part 2)][2]", Data was able to input commands *directly* into the neural net of a Borg cube, forcing all of the Borg on board to enter their regeneration cycle.  ***This demonstrates at least two types of interface at once: direct physical connection from Data's positronic brain to other devices (Locutus' Borg implants) and Data's connection, in turn, to the Borg Collective (a "wireless" connection, if you like, over subspace).***

Also see: "[A Fistful of Datas][3]".

I also wish to explicitly counter the comment by "The Spooniest" made below the original question.  Data was not "designed to function as a crew member".  He was found by chance on Omicron Theta by Starfleet officers, and his own career in Starfleet came later.


  [1]: http://en.memory-alpha.org/wiki/Datalore_(episode)
  [2]: http://en.memory-alpha.org/wiki/The_Best_of_Both_Worlds,_Part_II_%28episode%29
  [3]: http://en.memory-alpha.org/wiki/A_Fistful_of_Datas_(episode)