This is actually very straightforward, once you know where to look for it. Open up a version of Visual Studio into which you've installed the .NET Reflector add-in. Now on the main menu bar click .NET Reflector > Choose Assemblies to Debug (figure 1).
Figure 1. Bring up the "Choose Assemblies to Debug" dialog from the .NET Reflector menu.
The .NET Reflector Pro trial dialog will appear as shown in figure 2.
Figure 2. Click Activate in the trial dialog.
At this point just click Activate. The activation dialog shown in figure 3 will appear.
Figure 3. Enter your serial number, then click Activate.
Type or paste your serial number into the Serial number field, and then click Activate. You should see that, after a short delay, the dialog shown in figure 4 will appear.
Figure 4. .NET Reflector Pro has been activated.
Now just click Close and you're ready to go.