Hi Richard,
Try again, this time after you browse and provide the PST file to use, un-select the box "Import into Account"
The credentials you provide in the previous step (hostname, port, ssl, username & password) will be enough to push the selected PST into that Zimbra account.
Also watch out for importing large (10MB+) emails/attachments you will need to modify windows regedit to allow for that; after backing up regedit of course.
Feel free to take a look at the walk-through I use for this.
Importing a PST into Zimbra | DataSync University TL