The info you need is on that link I posted but here's the bit you need specifically:
Creating a Smart Folder
Make sure the Finder is the front most application, either by opening a Finder window or by clicking on your Mac's Desktop. We will recreate the Today Smart Search from the pre-Lion Finder sidebar as an example.
1. From the Finder menu, select File, New Smart Folder.
2. A Finder window will open, with the search pane open.
3. Select the area to search; for this example, click the This Mac item.
4. On the far right side of the search pane, click the plus (+) button.
5. The Search Criteria area will display, showing various buttons and fields, depending on the search criteria you chose.
6. Click the first search criteria button and select 'Last opened date' from the drop-down menu.
7. Click the second search criteria button and select 'Today' from the drop-down menu.
8. Hold down the Option button and click the '…' button to the far right of the search criteria you just set up.
9. Two new search criteria rows will display.
10. In the first new row, set the single button to 'None'.
11. In the last row of the search criteria, set the first button to 'Kind' and the second button to 'Folder.'
12. The search results will display.
13. Set the search order to Last Opened by clicking on the Last Opened column in the search results (you may need to scroll to see the column).
14. The finished Smart Folder search criteria should look like this (I've placed single quotes around the button text):
Search: 'This Mac'
'Last opened date' is 'today'
'None' of the following are true
'Kind' is 'Folder'
15 Save the Resulting Search as a Smart Folder
Click the Save button on the far right of the search pane.
Give the Smart Folder a name, such as Today.
You can leave the 'Where' setting at the default location.
Place a check mark next to the Add to Sidebar box.
Click the Save button.
The Today item will be added to the Favorites section of the Finder Sidebar.
