nvm found out! So I decided to turn this into a noob tutorial for all the others who might get stuck.
1. Download the AdMob FE addon. It should be a file with the ending ".c2addon"
2. Open Construct 2
3. Drag and drop the .c2addon into Construct 2. It will now prompt you to install it.
4. If you approve the install, you must restart Construct 2 before the addon becomes available.
5. Open your project. Go to the layout where you want to insert the ad.
6. Double click anywhere on the layout. A windows shows up. The new addon is under "Frosty Elk"
7. Go to the event sheet of the layout where the interstitial ad will show. Click "add event" and select Admob FE.
8. "Create full screen Ad".
4. Now read the rest of the original post.
*Hmm... the ad won't really load. I set my app to go to the next layout once the interstitial ad loaded. But, it is stuck because it won't load. Followed all the instructions.