Hi! Sorry for the late reply.

At the bottom of the extension's preferences page, you will see a header titled "Add a new search engine". To add a search engine, you need to fill in the line right below that header.
- if the first checkbox is checked then the search engine will appear in the context menu;
- to the right of the above checkbox is a textbox where you need to specify the name you want to give your search engine;
- followed by a keyword which you will be able to use in the address bar (i.e. omnibox). The way this works is that, in the url address bar, you start by typing "cs " (for Context Search) followed by the keyword you chose for the search engine you want to use (e.g. "ddg " for DuckDuckGo) followed by the search terms. So, if I want to search DuckDuckGo for "SpaceX", then I can type "cs ddg SpaceX" in the url address bar and hit enter;
- followed by a another checkbox which specifies if you'd like to use the added search engine to your set of multi-tab searches. What does this mean? Well, when you're using the context search list mode (i.e. not the icon grid), then you may have noticed that there's an item named "Multiple tabs search". When you click on that menu item it will open a new window with multiple tabs, where each tab will contain the search results of a search engine that has been checked using this checkbox. To summarize how to use Multi-tab search: you first check all the checkboxes (in the 4th column) that correspond to the search engines that you want to include to perform multiple searches in one go. Then, you click on the "Save Preferences" button and now you can use the "Multiple tabs search" from the context menu;
- in the last textbox on the line, you enter the search query string for the search engine you'd like to use. Here, you have the option to use %s or {searchTerms} in your search query string when the search terms are not located at the end of the url.
Once you have entered the above information, you can test whether it is valid and working properly by clicking on the "Test Search Engine" button. When you're satisfied, you can finally click on the "Add Search Engine" button.

I hope this helps. Let me know if you have any further questions.