Use this as an example on how to start the virtual console without the need of Java Web Start or accessing it from the web interface.
You can use the user and password that you use for the web interface.

You need an old JRE... I used 1.7.0_80 from the Server JRE package, also I have tested successfully 1.7.0_79 with MacOS.
You don't need to install it, just extract it or copy the files in "jre" folder.

Open the viewer.jnlp file that you get by launching the virtual console from the web interface with a text editor.
Note the urls to the jar files. Download the main jar file avctKVM.jar and the libs for your operating system and architecture.
Extract the dlls (.so Linux, .jnilib MacOS) from the jar libs.

If you don't see the MacOS libs in the file make sure you download it from MacOS.

Edit the bat/sh file according to your needs.

The file structure should look like this:

start-virtual-console.bat (.sh if Linux/MacOS)
avctKVM.jar
jre/<jre home here>
lib/avctKVMIO.dll (.so if Linux, .jnilib if MacOS)
lib/avmWinLib.dll (.so if Linux, .jnilib if MacOS)