Accounts Plus does not support estimates. You will need to go up to the next level which is Pro
Not sure about Accounts Plus, but have a look at this, it could be a work-around.
Open a blank invoice using any template, now go to "Edit" and have a look and see if there is an option called "Mark this invoice as pending".
If you have this option you can use it instead of an actual Estimate. Set up an invoice template and change the name to "Estimate". There is an option were you can tell it to mark the invoice as "Pending" make sure this is turned off. Now just 'mark the invoice as pending and it will be the same as an estimate just has a different name.