MiSTer board can be access through on-board Ethernet port. System has FTP, SSH, SFTP services running.
User name: root Password: 1
By default, DHCP is used to acquire IP address for the board. You can find it from your router (easier way), or from console connected to the board using the command ifconfig
If you need to set up static IP - please follow Linux manuals (/etc/network/interfaces changes)
mc (midnight commander) file manager is available for easier folder navigation.
The root of SD card: /media/fat
Related: Serial Console Access