The Look Up menu, on the Merops: Reviewing tab, has a list of popular search websites, including search engines, journal databases, online dictionaries, and encyclopaedias. To use these, simply place your cursor within a word, or select a phrase in your document, then click the website button that you want to use. This will search for that word or phrase using your default internet browser.
You can choose which websites are shown on the menu, using the Look Up page of the Merops Options dialog box.
Simply check a box to the left of the website, and it will appear in your Look Up menu. If your website isn't listed, click Add… at the bottom of the options page, and on the dialog that appears, follow the instructions carefully to add any website you choose.