1. If the markup item is an inline item, you can centre it by selecting the paragraph that contains it (press right-arrow when the markup box is selected) and choosing center-align from the Paragraph Inspector.
2. If it's not an inline item (which might be the case here), you could place it inside another box and centre the box that contains it by choosing Center from the Align popup in the Margins and Alignment section of the Box Inspector.
The reason there aren't any specific style options for markup items is because they're added without any additional wrappers (boxes) to which the style information can be applied. If you want to apply style/positioning information, you can do this by adding your own wrapper (step 2 above).