Do not select the generic "B737-700/800." Look for the entry named: "PMDG 737-800 (Exclusive Studio Tune)" or similar. It will have a gold or diamond icon next to it.
If you are a casual flyer who uses external views most of the time, the standard FSRealistic profile is sufficient. However, if you are a who sits in the left seat for 2+ hour flights, the FSRealistic PMDG 737 Profile Exclusive is non-negotiable. fsrealistic pmdg 737 profile exclusive
Click download. Once installed, click "Set Active." The UI will flash to confirm. Do not select the generic "B737-700/800