SPElement’s Remote Explorer for SP uses web services to access & view SharePoint internals. When accessing a local SharePoint instance knowing what’s the username/domain is simple, however finding out your SharePoint Online is a bit more tricky.
The username you are using to logon to SharePoint Online is not the real domain username that you should use. To find out the real user name:
1. Enter SharePoint Online
2. “Site Actions” -> “Site Settings”
3. On the left, in “Users and Permissions” -> “People and Groups”
4. Select the desired user
5. The “true” user name is under “Account”, under the format [domain]\[username]