Unfortunately this is not included in Menu Writer v1. It was added in v2 of the product. You will have to manually set the text to center using the menu.css file.
Find the block of code in the menu.css file that corresponds to your menu that is similar to this:
ul#cssmw {
cursor: default;
list-style-type: none;
margin: 0 0 0 0;
padding: 0 0 0 0;
}
Note: the code inside the curly brackets may be different. The ul#cssmw may have a number appended to it as well.
Add this in somewhere:
text-align:center;
so it looks something like this:
ul#cssmw {
cursor: default;
list-style-type: none;
margin: 0 0 0 0;
padding: 0 0 0 0;
text-align:center;
}