To open the Reference dialog from the beta cluster: Open VE, place your cursor anywhere, and choose
Insert > Basic reference.

To open the dialog from test.wikipedia.org:

1. Open VE, place your cursor anywhere, and choose Cite to open the Add-a-citation dialog.
2. Select the Manual tab and choose “Basic form.”

At the bottom of the dialog is a button labeled “Use an Existing Reference.”

But this is an option that the user has already rejected in favor of opening the current dialog with the intention of typing in a new reference. In set-ups where VE has a Cite menu (as on test.wiki), the user bypasses the Re-use tab and instead uses the Manual tab (Basic form option) to open the current dialog.

In set-ups without a Cite menu (as on the beta cluster), the user bypasses the Insert menu’s Re-use Reference option and chooses Basic Reference to open the current dialog.

So there’s really no need to repeat the “Use an Existing Reference” option in the current dialog. (Well – I guess you could argue that it gives users flexibility on the off chance that they arrive at the dialog by mistake, but to be consistent you’d ALSO have to add a button for the third option, Automatic – at least in set-ups with a Cite menu, where the basic reference dialog is reached via the Add-a-citation dialog with its three options of Manual, Re-use, and Automatic.)