Just enter the keywords you want to search and press the return or enter key to start the search. Clicking the icon at the left of the search field will open a popup menu where you can select the search engine that is used for the search. Also you can select one of the last search terms from here, because iCab will remember a small number of searches so you don't need to enter the same search terms again and again.
If you've removed the search field from the Navigation toolbar (using the menu item View > Customize Toolbar) you can open the search field using the menu item Edit > Find > Find in Internet (keyboard shortcut: Cmd-Opt-F) nevertheless.
For example, enter the following when you want to search for "iCab" using the search engine google:
g iCabIn the Searches panel of the preferences dialog, you can configure the key letters for each search engine (see below).
The internet search engines that can be used in the search field or the URL field can be configured in the preferences dialog in the panel Searches, so you can add new search engines or remove existing ones you don't need. You can also change the key letters that will be used in the URL field to identify the search engine.
Adding new search engines can be directly done in the preferences dialog, but this is a task for an experienced user because here you have to find out all the necessary parameters of the search engine yourself and enter them in the right place in the preferences dialog. A much better and easier way to add a new search engine into the "Internet Search Engine" list is the following:
Open the web page with the search engine you want to add to the list in a browser window. Now control-click (right-click) somewhere within the search form (usually it's the best to hit the "submit" button when control-clicking of the form so you don't miss the form) to open the contextual menu and here select the menu item Add Form to Search field. Then iCab will collect all the data for this form so that it can be added to the internet search engine list. The data is displayed in a dialog box, here you'll usually only need to define a key letter and you may change the default name for the search engine.
Note: If you didn't hit a form when control-clicking in the web page, the menu item "Add Form to Search field" is missing in the contextual menu. And because many web pages do contain many forms, and often even invisible forms for internal usage, you should control-click the submit button of the from you want to add. The submit button can be easily identified and so the corresponding form can be found as well.