That form should be submitted by the date on the notification. This homepage also has a timeline for each semester. Students peruse the sites in their concentration and choose 5 or 6 sites to which they send their resumes. You decide who you want to interview and make an offer. There is no guarantee.
Sites compete with each other for interns.
Yes, students receive 3.0 credits for the internship. Keep in mind that students are paying tuition to be able to work with you. Whether a student receives a stipend is between you and the student. Since there is a great deal of competition among sites for students, the sites that pay are preferred and sought out by the students. Students must be in their senior year of undergraduate studies or in the MBA program.