1- Select text inside the circle and on control panel: paragraph formatting controls apply Align center.
2- With the text still selected Object menu>text frame options>general>vertical justification>align:center.and you can also play with offest settings on base line options tab.
A lot depends on what you mean by centered, the number of lines of text, the actual font, and whether there are descenders.
Using vertical justification set to centered may or may not work for you, depending on the above factors, as it depends on the baseline, I believe, rather than the acutal glyph dimensions. You could try adding baseline shift, I suppose, to make fine vertical adjustments, or you can bite the bullet and use the old-fashioned method of two frames.