Dan,
The formula syntax to use depends on your source data. See this blog post and the user comments for more info:
http://mappingcenter.esri.com/index.cfm?fa=ask.answers&q=533
I don't recall if the contour layer in the LGIM has fields set up for coding index contours, but if not, create fields of short integer type based upon the index intervals you want to use. Then use the formula in the link above to select those contours that fit that interval, then code the index field with a value of "1" to indicate the particular contour line is an index contour. Otherwise the field has a value of "0".
For example, I have fields set up in our 2 ft. contour layer for 50 ft. and 100 ft. index contours. If I want to show 50 ft. index contours, I copy/paste the contour layer and write a definition query where Index50 = 1 (Index50 is my field name), then symbolize it as an index contour and label that layer only.
