The KNIME Nodes require two kind of licenses:
A license from Infocom Corporation enabling the nodes. This is a file with a ".dat" extension.
Save the ".dat" license file to an arbitrary location on the file system
Open KNIME after the nodes have been installed
Go to: File / Preferences / KNIME / JChem
The ChemAxon licenses should be installed the same way as for other ChemAxon applications.
If you have already installed the ChemAxon licenses (for usage outside of KNIME), there is no need to install them again.
The license can either be a ".cxl" file, or a license key to be used with a license server.
NOTE: The license key + license server usage requires version 4.0.0.v200300 (which includes JChem 20.3) or later.
Please visit this page for more detailed information about licensing ChemAxon products.