Enter the known letters into the top right windows, enter a <space> or "." for the unknown letters and then click the Search button. The results that match your search criteria will appear in the lower right window.
If you want to find a phrase then do the same as above but also include <space> or ". " for the spaces or hyphens between the letters. Click the Only Phrases radio button and then The Search button.
For anagrams simply enter all the letters you want to use in the window (order doesn't matter) and then click the Anagram button.
Remember that the less information that you give then the longer it will take to find the results of your searches.