The upgrade to Pinnacle Studio 17.5 includes a serial number for Sonicfire Pro 5.8 as well as a Studio Plugin activation code. These codes will be in the registration email that you received wtih your Studio 17.5 purchase. You must use both the serial number and plugin code that are provided in the email, otherwise the plugin code will not validate.
After you have installed Sonicfire Pro 5.8, you will unlock it by using the serial number that was supplied in the Studio 17.5 registration email (the serial number will begin with the letter P). Then apply all available updates to install the free tracks that come with the program. Once complete, click the Help menu and choose Plug-In Activation and select the "Avid/Pinnacel Studio" option. Then enter the Studio Plugin activation code (it will begin with "AVS...") into the field at the bottom to unlock the Studio plugin functionality.
You will invoke the plugin by clicking the SmartSound icon above the Studio timeline. This will create a temporary connection to Sonicfire Pro. You can then select one of the owned tracks (the ones with blue icons next to them) in the Express Track window, enter a custom length and variation, and click on the Send button to send the audio to your Studio project. You can then make additional length adjustments by shortening or extending the track directly on the Studio project.
Additional information is available in the Sonicfire Pro Help documentaion, accessed from the Help menu.