App Inventor apps do not run in the background, so you can not trigger the alarm when the app is not running.
You might use Eclipse or Android Studio instead of App Inventor. App Inventor is an entry level Android compiler intended primarily as a teaching tool.
We want you to have fun with App Inventor so here are some resources to help you learn to use the AI2 tools A very good way to learn App Inventor is to read the free Inventor's Manual here in the AI2 free online eBook
http://www.appinventor.org/book2 ... the links are at the bottom of the Web page. The book 'teaches' users how to program with AI2 blocks.
Does this help you program? Please continue to ask questions here however if you read the information above, you will not have to wait for answers.
Just try some blocks and have fun.
Regards,
Steve