Are you sure you logged into Impulse with the same account? The registration key is associated with your email address/account, so logging in with it should list Entrenchment in My Games.

You'll have to email [email protected] then

So, if I get a new computer. And I have already purchased Entrenchment..........Can I redownload from my E-mail? or is it already in my account? Being that I am using XP, and when I switch too my new comp Windows 7, will be my new OS. Will the game be in my account?

This happens a lot when the base game is purchased with one email address, and the expansion is purchased with a different one.  This happens often with Paypal, and is generally easy to remedy. 

First, go HERE and see if it's actually registered to your account.  If it is, try closing Impulse and running the following file:

http://sd.stardock.com/curthendzell/fix_registrations.reg

Make sure to follow the link in Internet Explorer, as other web browsers will not handle the file type correctly.  Once you follow the link, save the file and double click it to apply it.  Then relaunch Impulse.  You will need to re-login with your username/e-mail (either will work) and password.

If it isn't, and you know the email address you used, you can merge the two accounts HERE.

If you don't know the email address you used, or you no longer have access to one or both of the addresses, give [email protected] a ring, and we can help you out.