Go to The OpenLedger DEX and on the right you will see tab 'Find Markets'

In the exchange, select the desired currency (BTC, BTS, USD or OBITS) on the right side and click on the FIND MARKETS that is right under the navigation bar. 

Type in your desired currency in the search box and click on it. There you have a currency pair exchange in front of you where one of them is desired currency. On that page you can see all buy and sell offers. Choose a suitable offer and make your trade.