Beginning version 4.2 Paymee is simply registered with Excel via the Paymee Addin file ( PaymeeAddinV4.xlam ) being placed in the Excel Startup folder (the Excel startup folder specific to all PC Users - varies with Excel version).
Check your Paymee version - refer to the Install screen or see 'About Paymee'. If not running version 4.2 or newer, please download and install latest version 4.
If the issue persists, please re-install using these exact instructions.