I am having the same problem.  I bought new computer and wanted to come up quickly with Nitro Pro by downloading a Trial copy, knowing I have allowance for 2 Active installs.  I cannot figure out how to convert the Trial version to my active license.  When I enter the license number from the old install, I get "Unable to locate the Serial Number".  Of course this is off hours and there is no human contact.  The Email form forces me back to the Active Screen where now I am in fricking loop.