Look in the upper left for "User CP" It's the first one in the dark blue bar right above "DIS Home" which is in the light blue bar. (Note: Your colors may be different depending on your board color choices.)
Click on "User CP". Then look to the left, and you will see a choice "Edit Signature". Click on that, and you will have the area to insert your code. You can get a preview to make sure it is working before you leave that area. Save the changes, and you should have a countdown timer in your sig. There are some that are not allowed on DIS, but I'm not sure which those are.
Hopefully, someone who knows more about it can help if this doesn't work for you.