The view quality slider shoud be on the info bar by default see the screenshot below. I'll take a look at your file to see if there is anything else that may be causing the problem.
Edit: when I tried to open your file in Designer Pro 7 I got an error I have never seen before, I'll attach it here as it may be helpful to the Xara developers. If anyone else has difficulties with opening this file it may be a good idea to send the file to Xara support.
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