here is what you need to do:
sudo nano /etc/fstab
put a line like this:
//192.168.1.17/Airdisk/ROMS /home/pi/RetroPie/roms cifs sec=ntlm,username=youruser,password=yourpassword,_netdev,dir_mode=0777,file_mode=0777 0 0
replace my IP and path, user and password to your config.
sudo mount -a
you should see your NAS mounted to Retropie roms folder now.