The complex search screen can be opened by using the "Images-->Find-->New Complex tag search" menu.
The following screen will be opened:
Filtering is done by dragging the tags from the application's tag tree structure, You will notice that there are 3 parts to that screen:
1) With(And): here you can place all the tags you wish to filter by, using an "And" relationship between them.
2) With(Or): here you can place all the tags you wish to filter by, using an "Or" relationship.
3) Without: here you can place all the tags to wish to exclude from your search.
Let's say we wish to find all images tagged with a 'Blue' tag. simply drag the 'Blue' tag to the 'With(and)' row and press the 'Execute Icon':
Now, let's say we wish to find all images tagged with a 'Blue' tag as well as images tagged with a 'Red' tag. Simply drag both the 'Blue' tag and the 'Red' tag to the 'With(And) row and press the 'Execute Icon':
Now, let's say we wish to find all the images tagged Only with a 'Blue' and a 'Red' tag. Simply drag the both the 'Blue' tag and the 'Red' tag to the 'With(And) row and drag the 'Other' tag to the 'Without' row, (the 'Other' tag can be any tag from any level)., now press the execute icon:
These are pretty much the basics of the Complex search feature.
Tip: you can use the 'Save' icon in order to save your query and run it directly from the "Images-->Find-->Saved queries" menu.