Within LPCXpresso, I go to Help->Activate->Create Serial Number and Register (Free Edition), copy the serial number to clipboard and go to the activation site, but the site can't be found. I've tried on multiple computers multiple times throughout the past 24 hours. Am I missing something?
The site certainly seems to be up and running right now. Are you still seeing problems?
More information on activating the LPCXpresso IDE product can be found at: https://community.nxp.com/message/630539
However, you might want to instead consider trying the MCUXpresso IDE product which has replaced LPCXpresso IDE. MCUXpresso IDE requires no activation to use the Free Edition. For more information, visit : http://www.nxp.com/mcuxpresso/ide
MCUXpresso IDE Support