There are 2 settings that need to be changed in your dynmap configuration.txt, this file will be in your servers install directory under plugins/dynmap
If you are not sure where your install directory is, first check what JAR File you have set on the main page of Multicraft, if your JAR File is Spigot, the install directory will be Server_Spigot, if your JAR File is craftbukkit, the isnsall directory will be Server_CraftBukkit.
The first setting you will need to change is deftemplatesuffix
deftemplatesuffix: lowres or deftemplatesuffix: vlowres
This is required due to the the extreme amount of storage that Dynmap uses, if it is not changed it is very likely that your server will go well above the soft limit of 30 GB of disk storage for your server.
The second setting is webserver-port
This will need to be set to one of the 5 ports above your servers port, for example, if your server port is 25565, you can use any of these:
You also want to make sure your IP setting is either commented out or set to 0.0.0.0