You can find your computer ID under Start->all programs->national instruments->NI Licenses

 Click on "display computer information"  

Get your computer/machine ID (as outlined in  "Where's my computer/machine ID?").
 
Go to the website http://sine.ni.com/nilg/jsp/customer_activate_initial.jsp
 
Put in the software you have and the correct version. Make sure this information is correct.
 
Click continue and the the next page will have the activation code at the bottom.

Go to Start->all programs->national instruments->measurement and automation explorer
 
Click on the Software tab. If your software is in the list and doesn't have a red X by it then the software is installed.  

First you have to generate your activation code (as detailed above). There should be a radio button that says "apply a 20-character activation code previously acquired for this computer". Click next. Cut and paste or type in the activation code that you generated before.  

There is a way to activate the software by simply starting the License Manager. Click activate. Select "activate automatically through secure internet connection". Follow the prompts and it will activate automatically.

Make sure you have the correct serial number and make sure you're typing in the correct activation code. If that's correct and you've tried both ways of installing documented above then contacting NI would be a good next step.
 
Here is their support website: http://www.ni.com/support/