The Bitstamp module will default to trading on Bitstamp's EUR market if your machine uses a currency other than EUR or USD. (If you use USD, it will default to the USD market.)
If your machine is configured for a currency other than EUR or USD, and you wish to trade on USD instead, you may change the default behaviour by running the command below in your server:
curl -sS https://raw.githubusercontent.com/naconner/lamascripts/master/switches/bitstamp/bitstamp-usd.sh | bash
Then, ensure you have a balance in the USD account and place a test transaction that is above Bitstamp's minimum purchase amounts.