The credit is a calculation based on the difference between EI premiums in 2012 and in 2011. To be eligible for the credit, you must have, in 2011, $10,000 or less in employment insurance premiums. If you meet that criterion, when the employer's T4 information return is processed, there is an automatic calculation that looks at the difference in the amounts.
On November 5th, 2012. See this statement in context.