Edit my application previously selected item should be viewed?

Don't use set_checkbox() to change the value of the checkbox, just it's checked or unchecked status!
You really should start reading the documentation!!

And once again: it won't work of you give elements the id="mycheck1" in a control structure like foreach  { }. If there are 10 checkboxes, they all have the same id. How would that help you to find the right one?

So, forget the whole id part. Use a css class to handle the checkboxes through jQuery, and an array name like "certid[]" to return the values of the checked checkboxes to your controller after submitting the form.

If you're going to use form_validation, use the "certid[]" reference too, including the square brackets. There are lots of examples for that on the internet.