The BlinkUp SDK is limited to folks who have a commercial relationship with Electric Imp, ie. customers who pay for our service. This is because customers need the SDK to create the apps that they will supply to the folks who buy their products.
Simply, the SDK is not needed for development, so it’s not made available to folks with free developer accounts. Developers use the Electric Imp app to get their development devices linked to their accounts.
If you need to use the SDK in order to build an app that you can distribute to end-users, then you’ve reached the limit of what your developer account can bring you and you need to enter into a commercial agreement with us.