I have a small business. I contributed $1000 towards SEP IRA for year 2014. However, the transaction was done in January of 2015 using the retained earnings of 2014. Obviously, this money is tax deductible for my business.

What is the correct way of posting this transaction so that it shows up on my PNL statement for 2014?

You can make a balance day adjustment accruing the expense in the 2014 year.