Hiya dawn,
In the 'Box' tab in CSSMWriter's 'Edit Theme' settings, select the menu level and then 'Item' in the section 'Dimensions, Margins and Padding'.
You can tweak the padding (in your case the top and bottom padding) to centre your text vertically.
If the text appears too high in the menu, you can increase the top padding. To keep the menu height at 45px, remember that if you add x pixels to the padding, you will have to deduct x pixels from the height.