The simplest way to perform a search based upon a specific biblical text is to start with the Scriptures button at the top of the ATLA window.
The Scriptures button will take you to a list of all biblical books in canonical order. Clicking on the name of the book to search. This will make sure your results are about the Gospel of Mark, not just written by someone named Mark or about physical marks. You can also click on Expand next to any book to search at the chapter level or expand at the chapter level to search individual verses.
If you want more advanced search strategies based upon biblical texts, here is a video produced by ATLA.