Your first step is to open your order confirmation email that contains your serial number.
Highlight the serial number (it starts with SBSE) and copy it to the clipboard (hold Ctrl and C).
Run SyncBackSE. If the serial number entry window appears then your serial number will automatically be taken from the clipboard and used. You then only need to click the OK button to license the program.
If, instead, the main window appears (e.g. because you've asked SyncBackSE not to prompt you to enter a serial or you've already used a different serial number), then select Enter Serial Number or Change Serial Number via the I would like too... button. If you're using V8 or older, select Help -> Change Serial Number from the main menu. You can then enter the serial number (it will be automatically taken from the clipboard).