dot is part of Graphviz (http://www.graphviz.org/About.php). You need to install Graphviz and then modify your PATH so Windows (what I assume you are using) can find it. "the command pyreverse generates the diagrams in all formats that graphviz/dot knows." (http://www.logilab.org/blogentry/6883.)
Once installed dot executable can be found in
C:\Program Files (x86)\Graphviz2.36\bin\
To make sure it was installed properly run the comand
C:\Program Files (x86)\Graphviz2.36\bin>dot -Txxx
And you should see something like: "Format: "xxx" not recognized. Use one of: bmp canon cmap cmapx cmapx_np dot emf emfplus eps fig gd gd2 gif gv imap imap_np ismap jpe jpeg jpg metafile pdf pic plain plain-ext png pov ps ps2 svg svgz tif tiff tk vml vmlz vrml wbmp xdot xdot1.2 xdot1.4"*
You can download Graphviz here: http://www.graphviz.org/Download_windows.php