The Search box is always visible at the top right corner of the window. Click inside the box, type a word, and click Go to perform the search.
The Search box will return results matching all the search terms which you enter. This means if you type Christmas tree, it will look for items which contain the word Christmas and the word tree.
Scran also supports Boolean searching, which allows you to use words like OR and NOT to perform complex searches.
More Simple Searching Ideas
Having performed a search, you can click on Revise Search to narrow your search by adding or excluding terms.