Did you fill out a form that you mailed or faxed? If so, there should have been an option to withhold taxes or not. If you did such a form, call the custodian back and ask that the rep find it and review it with you. If it was done online or over the phone, there's probably no help there.
You could just recharacterize it. That undoes the entire thing and you can try again next year.
I'm uncertain as to whether there's any other way to fix it. You could try calling the IRS help line and see what they say.
You didn't say when you did this. I'm assuming this year. If so, then the conversion has no effect on your 2012 taxes other than the 8606 to record the non-deductible contribution. You don't need to hold up your tax return any further. If you did the conversion last year, then you would need hold off.
BTW, there's no such thing as a non-deductible IRA. There's point telling the custodian that. There are only deductible or non-deductible contributions. That's between you and the IRS.