Learn English – word for “people who are computer illiterate”

Dysgraphia, per Dorland's Illustrated Medical Dictionary 31st Ed., is simply "difficulty in writing." Therefore, I disagree that this is a correct answer.

Agraphia is defined as:

"Impairment or loss of the ability to write; it takes two forms, one involving poor morphology of written letter forms and the other a reflection of the aphasia also observed in spoken language... Called also graphomotor aphasia"

I do think you need to carefully make a distinction between the person who never learned to write and those who have lost the ability to write. One may be a learning issue, whereas the other can be the result of a neurologic problem, such as a stroke. Agraphia and dysgraphia are terms that imply that the ability was once there, but is now either impaired or lost.

I don't know of a term that would cover the loss of the ability to write through disuse.

Limp-dick (or limp dick) is the most common slang term. (at least in US English)

noun An ineffectual man; an impotent man; wimp (1970s+)

^{http://dictionary.reference.com/browse/limp-dick}

There are a lot of entries in Urbandictionary also.