Actually, what OS are you running? How do you add the names?
DON'T CLICK ADVANCED!!!
I do it the following way in WinXP SP2:
1. Right Click on the folder from Windows Explorer
2. Go to Security tab
3. Click Add
4. Click Locations...
5. Choose my computer at the very top
6. Back at the select users and groups window to the left of the "Check names" button type "ASPNET" and press that button;
7. The name should get your computer name attached at the left and get underlingned.
8. Press ok.
9. Follow the obvious from there.