popup forms and namespaces

Question

Hi everyone! I'm learning c# and I'm exploring various c# namespaces. I've made a few simple apps testing WMI, pinging etc..so I thought I'd put them all into one big app together and just use a form with buttons to launch each app. The idea is when I want to use the ping utility I click on 'ping Utility' button and a form comes up with that app in it.  Strangely though even though the apps work as standalone c# forms, If I try to paste the code into seperate forms (i.e the pop up form) it complains about namespace name ManagementObjectSearcher could not be found. But I've included the same using directives in the popup form as the original standalone form.

I added the refence manually via references - right click, add reference, and this works. I don't understand why though you need to do this to add the reference, I thought that was the whole point of having 'using System.Management at the top of the form!

p.s Sorry in advance if this is a reallt stupid question, I am reading the books to learn, but haven't found anything that would help me with this yet.