In short: you can no longer, because this function is no longer available. According to Chris Webb :
The cube browser has now disappeared from the SSDT and is replaced by the control used by the SSRS query generator. This is because the old browser used OWC control, which was deprecated many years ago and finally had to be replaced; its a massive step backward because this control is really terrible: for example, you can no longer set anything other than column measures in your query.
You can confirm this by going into design mode and creating a simple MDX query with a measurement on the column axis:
SELECT [Geography].[Country].[Country] ON COLUMNS FROM [Adventure Works]
If I run this request in SSAS browser, I get an error
The request failed: ... The first axis [should not] refer to any size other than the dimension "
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