You must have at least Oracle Solaris 11.4 SRU33 installed. For example:
$ pkg info entire Name: entire Summary: entire incorporation including Support Repository Update (Oracle Solaris 11.4.33.1.0). ... <output omitted> ... Version: 0.5.23 (Oracle Solaris 11.4.33.1.0)
See Hardware and Software Requirements for more information.
# grep mlxne /etc/path_to_inst
If the adapter is properly installed, you should see output similar to this:
grep mlxne /etc/path_to_inst "/pci@2,0/pci8086,2f08@3/pci108e,3044@0" 0 "mlxne" "/pci@2,0/pci8086,2f08@3/pci108e,3044@0,1" 1 "mlxne"