Yahoo! Smart Tools are enabled by default for Yahoo! Toolbar 6.1 and newer for Internet Explorer. If you've disabled Smart Tools and want to turn it back on, do the following:

1. Click the toolbar's Pencil menu.
2. Select "Toolbar Options".
3. Check the box labeled "Enable Smart Tools when selecting text".
4. Click "OK".

Now you should see the Smart Tools icon in your browser window whenever you select text in Internet Explorer.