When did you brew this? If it's only been a couple of days, it's not too late to add the sugar.
I use just regular table sugar in my double IPA's. I use about 1.5lbs. To add it now, put your sugar in a pot, add in just enough water to dissolve the sugar (I only need like 10 oz, for 24 oz of sugar) Boil it on your stove for 2 minutes or so to kill any nasties, then cover and cool. Once it's cooled down you'll have simple syrup, that you can just poor right into your fermenter. The yeast will eat it all up.
I think most guys on this site will tell you that there's no noticeable difference between dextrose (corn sugar) and table sugar when used like this. I've used both and have noticed no flavor difference.
Personally I'd use a calculator to determine how much sugar to add up to about a 1.5lbs to get my desired O.G. and then add that, rather than play with the dry-hops.