Well there's two main places to set credentials for hosted virtual machines:
1) You add server credentials - this is done right when you add the ESXi or vCenter server to the group.
2) Specific machine credentials - once you see the list of VM's on your server you have to add them to your group, and then you can set credentials for each individual machine.
You can also set machine group credentials - this would apply for any VM's that you actually add into your group and don't set specific machine credentials.
I hope that explanation makes sense and helps.