How do I enter both a 2018 W-2 and W-2c from my employer the only change on the W-2c is an X in Box 13 for Ret. Plan which was blank on the original W-2?

Enter the info as a single W-2.  Enter all the info from the original W-2, and put the X in Box 13 where it belongs.

Thank you, Have you dealt with this in the past?  Should I mark the "corrected W-2 box" on the page following the W-2 form input or leave it blank since this becomes a combined W-2/W-2c?

Yes, you should mark that box.  That will tell the IRS that you took the W-2c into account when you prepared your tax return.
Yes, it is not totally uncommon for an employer to split an employee's W-2 information into two W-2 forms.

Thank you for your help TomD8!