There are a few reasons why an account may be placed on hold. 1) If you are signing up at the same address as someone else. 2) If you live at the same address as someone else. 3) Crosscheck of your identity shows inconsistency. Since Instant Rewards is a program that allow users to receive money without necessarily having to spend any money, there are tons of people who try to scam the system. Also remember you are only supposed to sign up one person per household. To prevent against this activity, Instant Rewards will typically ask for ID and a piece of mail if suspicious activity is suspected. To rectify this, you typically just have to send a picture of your ID and a printed piece of mail to email@example.com. This can just be a regular picture you take with a cell phone camera or other device, it doesn't have to be high quality, just readable.
As long as you don't delete the emails you receive when you complete the offers, you are fine. The emails you get back serve as proof that you completed the offer. Just log into the Instant Rewards site of the offer you completed, click on the "Support" tab and click on "submit a missing credit request." Once there, click on the tutorial of how to show full headers in your particular email account, copy the email of the offer with full headers, select the offer you completed, select the date you completed it, paste the email in to the designated location and submit the missing credit request.