serial code entry?

I recently got a serial code for serum, however, I don't know where to enter it in
if anyone knows how to do this, please tell me how and where.

When you launch Serum in it’s demo version I should present you with two options: Try and Buy.

Try clicking Buy.

From where did you purchase the key?

Please use the contact form if you purchased this from a legal reseller, so we can sort out your registration.