When I enter my customer number and password and then click on Design Site, I am taken back to log in page.
Make sure the clock is correct on your computer and not set for a future date.
When I enter my customer number and password nothing happens...it keeps asking me to enter my customer number and password.
Try lowering your browsers Privacy Settings as they may be blocking the cookies that are used to store your sign-in credentials.
On Internet Explorer you can configure Privacy settings by clicking Tools > Internet Options and selecting the Privacy tab.