Try entering the code again, double-checking that the License Owner information (including any [EDUCATIONAL LICENSE] text) and the License Code are exactly correct, with no spaces, tabs, or other characters added to either end. Next, double check that you’re not trying to enter an OmniFocus code into OmniOutliner, for instance, or an OmniGraffle Professional 5 code into OmniGraffle 5 Standard. (Of note: OmniFocus 2, OmniPlan 3, OmniOutliner 4 and later, and OmniGraffle 6 and later are now all-in-one downloads with both feature sets included in a single app. When you enter your license, the appropriate feature set will be activated.)
If those don’t work, send an email to firstname.lastname@example.org and we’ll be happy to be of assistance.