SPOILER ALERT!

im at the end of the game and i have to kill frank horrigan, if i try to fight him alone i just get fried by his turrets, so according to the walkthrough i have to activate the computer using the presidential access key, i have the key but when i use it on the terminal it just says "that does nothing", ive tried using the key from the inventory or equipping it on my person, then i tried logging on to the terminal witth the key equipped, nothing works, please help! i dont have the speech skills to convince the dudes in the back to help me, so this is the only way. Im at 160% small guns, have the MKII power armor and have 205hp, i can handle frank horrigan by myself, but those damn turrets.

If you tried hacking the terminal without success earlier in the game it might have shut down for good, so you won't be able to access it any more.

You'll have to fight both Horrigan and the turrets. In order to make this battle easier you can flee into a corner where the turrets can't see you to fight Horrigan alone or you can stand directly besides Horrigan so the turrets which hit you hit him too.

Use drugs before the battle, they do make a difference. 2 doses of Jet should grant at least 4 valuable extra action points for example. If you still have a cookie eat it.
You can also use aimed criticals to cripple Horrigans arms so he can't use his plasma gun any more. The monument chunk and psycho grant resistance to normal damage.

i didnt use the terminal earlier. I have a savegame right before i embarked for the island and i keep doing it over to make sure i dont do that. Basically using the Presidential access key "does nothing" and when i simply use the terminal, it allows me to log in but then it just says "back" or "i cant do this here"

There might be an int or science check involved for using the terminal, so take 1 or even 2 mentats to raise both before using the terminal and check if you get more options.

You can destroy the turrets when you arrive at the Enclave (the first time you enter that room).

Keep on trying. The turrets' chance to score a critical which ignores armor is not very high so with a little luck you can succeed. Reducing their hit chance by standing far away or having something in their line of fire will also reduce their chance to score a critical.

If the turrets are worrying you more than Horrigan you can kill them first, you should be able to disable several of them per round with aimed criticals.

Yeah, I know - necroing, but since there was no definite answer in this thread: You need 100 Science to use the presidential login.
Which is surprising, since it's not fancy hacking you do, just using the presidential access key to access the programs.