"How do I apply these presets to my project?"

Open After Effects < At top, click animation < Apply animation preset < Double click the presets you bought!

"Why is it saying "Missing: Plugin"?"

This is because for the required preset that you have bought, you need a specific plugin for it, i do not provide plug-ins.

"What happens if I bought the pack and I closed my tab?"

All packs and presets that you buy from my payhip should directly be sent to your E-mail.