liability threshold models for dealing more than 2 Categorite data

I am a green hand for OpenMx study. Although I took a "behavior genetics" course, and have some experience about using SEM to deal with continuous data in the practical. I do not anything about liability threshold model since yesterday. Now i need estimate the heritability of two variables which both contain more than 2 categorite. I want set up two univariate liability threshold model first, and set up a bivariate model. However, there is limited information for me to start. I read some articles about liability threshold models. Almost of them are talking about dichotomous data ( like case-control study), few about ordinal data. So I am wondering is anybody here could give me as a beginner some suggestion about using liability threshold models deal with more than 2 cartegory variable data (classical review, slides or example scripts will be helpful.)? If, so I will be very appreciated!