The login depends on if tour host is windows or linux. For windows you must enable your administrator account and set a password for it. For linux you must have a password set for root.
For your first login you must use administrator or root (as appropriate) once you are in you can set up other local users on the host to be able to use the gui. Having done that, you can disable the administrator/root if required.
I have not needed to use the Administrator account. I simply use my normal account with administrator privileges. My OS is below. I don't know about Linux.
I use my account user name and password I use to login to Windows.
Host and Guest Systems:
Windows XP Pro x64 Edition;
VMware Server 2.0.0RC1, Build 101586