According to their documentation, you can log in initially as:
username: dba
password: dba
Edit: i also found that there are two different default users setup:
There are two system users of immediate importance:
dba -- the relational data administrative account
dav --the WebDAV adminstrative account.By default each of these accounts has its password set the same as its username. It is strongly advised that you change these as soon as possible for obvious security reasons.
http://ods.openlinksw.com/wiki/main/Main/VOSUbuntuNotes#Using%20Virtuoso