Write letters to scifi celebrities on computer

I'm not sure you can do that. On the computer in Sims: Freeplay you have four options and those won't change unless you have a Mission that asks you to do something special on the computer, like for example I had a mission where I had to get a quote from an insurance company on the Sim's computer, but since completing that mission, the option to get a quote is gone. Only certain things can be done on the computer.