I configured my 2 firewalls for High Availability now I can't connect to my slave firewall anymore. What can I do?

You probably can't connect to your slave's webserver anymore but you should be able to login via SSH (if you have SSH enabled).
To connect to your slave firewall via web you can do the following:

(Assuming your management net is 1.1.1.0/27)

Add a virtual network interface to your machine and assign an IP address belonging to the management net to it. You can now directly connect to your slave firewall on its physical address 1.1.1.2.

On Linux you can do this by typing 'ifconfig eth0:0 1.1.1.6/29'  in your shell.

Note: Endian Firewall will split your management net into 3 small nets, one for the green, one for the blue and one for the orange interface. This means your green management net in this case would be 1.1.1.0/29 including the following IP range 1.1.1.0-1.1.1.7. Usable IP addresses would be from 1.1.1.1 - 1.1.1.6, where your master firewall will use 1.1.1.1 and your first slave will use 1.1.1.2.