To activate MOBILedit!, select the Activate Product item in the Help menu. The Activation dialog will open up. Make sure you enter all the Activation data correctly - Name, Company and the Activation Key must be entered in exactly the same way as you received it. Since version 7 you can enter just your activation key (without name or company)
It is recommended to use Copy & Paste (CTRL+C; CTRL+V) to insert the information from the purchase confirmation to prevent any mistakes. The Activate button enables when you enter the correct key.