Welcome to the forum! 
If you already have nvidia-390xx-dkms installed, then it seems you already do.
Either way, currently, MHWD does not support DKMS modules. If you want to use the precompiled modules (linuxXX-nvidia-390xx), then first remove nvidia-390xx-dkms that you apparently installed manually. One would only need the DKMS version if one was using a third-party kernel.
Please post your system information as outline here as well as mhwd --listinstalled