I'd like to do this as well, I cannot get the solution posted on NordVPN's site for MikroTik to work as the VPN isn't connecting although the traffic marking and routing does work (with other VPNs I've tested)
I can get NordVPN working only with L2TP/IPSec and only with certain servers, I was provided with three servers in the USA that work, trying to get details of European based servers that will work.
My setup works as follows:-
/ip ipsec peer
add address=18.104.22.168/32 comment="Nord VPN USA 494" dh-group=modp1024 enc-algorithm=3des exchange-mode=main-l2tp \
add comment="Nord VPN USA 494" connect-to=22.214.171.124 disabled=no name=nord-l2tp-out1 password=myNordPassword user=\
add action=mark-routing chain=prerouting comment="Nord Push client 10.1.1.100 through VPN" new-routing-mark=\
nordvpn-l2tp passthrough=no src-address=10.1.1.100
/ip firewall filter
add action=accept chain=forward comment="Allow outbound to NordVPN, you may not need this but due to my VLAN setup I require it" out-interface=nord-l2tp-out1 src-address=10.1.1.100
add distance=1 gateway=nord-l2tp-out1 routing-mark=nordvpn-l2tp
You can check the connection status with:-
/interface l2tp-client monitor nord-l2tp-out1
;;; Nord VPN USA 494
They recommended using the IP address of servers instead of DNS name. I also had issues with my email address and password so had to simplify the password to remove some extra special characters.
However the speed is not great, I'm hoping when I get a closer server to work it may help.