Well did you enable the VRDP server when you set it up? Because even when the extension is installed, the VRDP server is disabled by default. It can easily be enabled on a per-VM basis either in the VirtualBox Manager in the “Display” settings (see the section called “Display settings”) or with VBoxManage:
VBoxManage modifyvm “VM name” –vrde on
Also verify that you are using the correct port:
The actual port used by a running VM can be either queried with VBoxManage showvminfo command or seen in the GUI on the “Runtime” tab of the “Session Information Dialog”, which is accessible via the “Machine” menu of the VM window.
Aside from that check to see on your network if specific ports are being blocked and if so either have the ports you are going to use opened or use one of the ports that are already opened.