The answer that I gave received one up vote, so I can't understand that it is unhelpful - if it was down voted or ignored then maybe. How can I now get unbanned - I have only one answer which I must apparently improve, but I can't really make it that much better?
You have three deleted answers. All your answers posted promote one product without disclosure. This is considered as spam and abusive – under normal circumstances your account would have been placed in a timed suspension or deleted altogether.
See our help page:
Avoid overt self-promotion.
The community tends to vote down overt self-promotion and flag it as spam. Post good, relevant answers, and if some (but not all) happen to be about your product or website, that’s okay. However, you must disclose your affiliation in your answers.
I don't see a way for you to correct this other than trying to participate by asking good questions, proposing edits on posts, et cetera. There might be a way to get unbanned, but given that the specifics of the banning algorithm aren't known, it might not work.
If you want to promote XpressDox, you can buy ad space on Super User by contacting the Stack Exchange sales team.