Since version 6.5 you prefix the part of the expression which specifies the exclusion group, e.g.
NOT:*.dll;*.exe;*.sys;*.dat
With this syntax you can then also add additional criteria, e.g. to search for all files with the word Window
excluding PDF files you can enter:
Window;NOT:*.pdf
Or, to search for any image file with the word New York
you can type:
*.png;*.bmp;*.jpg:New York
There's a few more nice DOS Expression features since 6.5 so be sure you check out the help file:
Help: DOS Expressions
Note: In version 6.2 (or earlier) you could exclude multiple types by clicking the Not check box next to the File name: field and then specifying the types to exclude, e.g. the DOS Expression
*.dll;*.exe;*.sys;*.dat
This was changed with version 6.5 to allow more flexibility with DOS Expressions.