Just right-click on the selected text and choose
* Lookup 'text' on Merriam-Webster
context menu item and there you go!
It was easy, huh?
Or alternatively you can hit "Alt + W" shortcut to get the selected text looked up or if there is nothing selected, then Merriam-Webster online dictionary is started up on a new tab. (Since version 1.2)
HINT: Just be careful not hitting "Ctrl (Cmd on Mac) + W" instead, because it will close your active tab. But in case if you did so, don't panic, you can recover that tab by pressing "Ctrl (Cmd on Mac) + Shift + T".