In IE7 to use the developer tools you must install IE Developer Toolbar, then to open it go here:
Tools (on the right-hand side) > Toolbars > Explorer Bar > IE Developer Tools
(This is so hard to find I had to make a picture)
In IE7 and IE8, you may wish to take important note of two buttons on the developer tools window.
The first button will clear the browser’s cache (IE will normally hang onto js, css, and image resources unless you explicitly do this). The second is the Refresh button on the Dev Tools window itself – not to be confused with the main browser window’s reload button which will reload the page.