License key and activation code are two different things. I think you got a license key but no activation code. An activation code is an alphanumeric sequence consisting of five groups of six characters each, like this one: AB1C23-4DEF56-7GHI89-JK1L23-MNP456. A license key is an .xml or .zip file.
Please first check on Tools & Settings > Plesk > License Information > Additional License Keys if your add-on license is not already listed there.
If it is not already shown there and you have no activation code, but instead a license key file in .xml or .zip format, then please enable uploading license files by adding this section to your panel.ini file (normally located in /usr/local/psa/admin/conf/panel.ini):
Code:
[license]
fileUpload = on
Afterwards you will be able to upload your license key file to install the license.