Richard -
There are 2 challenges on a Meteohub system.
The first is when you connect via a serial connection (which you cannot); root / meteohub
The second is the username / password on the web interface, which is meteohub / meteohub.
If neither of these is the solution to your problem, please explain further, in particular this bit:
On the PC is a piece of software called VWS (Virtual Weather Station) which is no longer functional as it can't find the data file which is on the Sheevaplug/SD card