I guess you want to connect to a VirtualBox guest running on another machine and running with the VRDE extension enabled. This extension enables connections to the guest console through the VRDP protocol (VirtualBox Remote Display Protocol). So you need a VRDP client.
VRDP is essentially the VirtualBox implementation of Microsoft RDP, the Remote Desktop Protocol. So an RDP client should work to connect to a VirtualBox guest having it's VRDP port enabled.
In the UNIX world, one such client is rdesktop. It is available in the usual Linux distros, for OSX and the other *BSD variants. Another alternative, if you're using KDE, is
krdc, the KDE RDP client. Check your package manager.
On Microsoft systems, you should be able to use the Microsoft Terminal Services Connector or
All those RDP client take one mandatory parameter: the host to connect to (as an IP or as a hostname) and one optional parameter: the port to connect to on the guest host (usually defaulting to 3389).
If it's not what you're looking for, could you make your question clearer please?