You have to complete the hacking minigame 25 time to unlock this.
If you're having a hard time solving the password, use this solver to compute the correct password:http://kennelbound.com/fallout3/
There are not nearly as many terminals in the game as there were in Fallout 3 and the vast majority seem to be scattered throughout buildings.
Some terminal locations, taken from a forum on x360a:
Vault 11 - 2 (1 easy, 1 hard)
Camp Golf - 2 (1 easy, 1 hard)
Goodsprings- Schoolhouse and Prospector Saloon
Primm- The Bison Steve Hotel
Sunset Sarsaparilla Headquarters- Theres about 3 or 4 in there.
NCR Embassy on The Strip- 2 in there, 1 in the building on opposite side
Crimson Caravan Company- I believe there's one in main office
Black Mountain- two in the bottom level of the radio station, 1 is in the prison building but its a very hard lock
REPPCON HQ- 3 on first floor, one is very hard. second floor has two, one is behind a locked door, top floor has two, one is very hard.
Hidden Valley Bunker- at least three or four in there.
Helios One- One in the solar collection tower
Vault 3- At least 4 or 5 scattered around the various levels
Camp Searchlight- 4 or 5 located around the friendly turrets. One is a very hard lock.
The following skill books will help you boost your Science skill if it is lower:
Brewer's Beer Bootlegging: In the basement of the shack, on the table with the vial experiment, opposite the still.
REPCONN Headquarters: Inside the headquarters, on a shelf inside the Gift Shop storeroom, near the Rocket Souvenirs.
Nipton: On the mayor's desk, top floor of the Town Hall, next to the Programmer's Digest magazine.
Camp Forlorn Hope: Inside Major Polatli's tent, in the far right corner, on a desk.
Having 100 science will make the hacking of easier terminals quicker, as less options will be present, and will allow the hacking of Very Hard terminals.
You can check your progress in the Pip-Boy by going to the Misc section of the Data area, then hitting Y to view the challenges, then moving the the Other Challenges page. The pertaining challenge is called Healthy Glow.