I was playing around with Eclipse and TestNG, the thing you need/want to do is, after you have installed testng via de Eclipse market place and restart Eclipse to go to the project build path.
So right click the project and select from the popup menu Build Path » Configure Build Path...
In this window, make sure you have selected the Libraries tab in the right-hand side of the window.
Next select Add Library... and from the resulting window that pops up select TestNG (or JUnit).
When you select Next or Finish (depending whether you picked TestNG or JUnit), you will then see TestNG under the JRE System Library entry as another library entry. If you expand this you see the testng.jar being included and pointing to the right jar file that's in Eclipse's plugins directory.
When you now press OK you should see the imports getting resolved.
You will need to remove any external jar dependencies for TestNG or JUnit of course, because it's double and will most likely lead to problems.
Now, when you go to Run » Run Configurations... you see a TestNG (JUnit) entry. When you select that entry and create a new configuration underneath it, it should already resolve everything you need (classes, packages, and so on).
Oh, do keep in mind that you will have to mark the test folder as a source folder for it all to work. Right click the folder, select Build Path » Use as Source Folder.