Related tables are not supported in the current version of ArcGIS Mobile. But they may be supported in the future. There are two solutions thought:
1) Instead of using a related table, use a FeatureClass instead, in which you can make one, point to be a dummy, and then use it as a table.
2) Search here on the forums on how to implement the related table workaround. I believe there are one or two posts on how to do this.
I personally just went with the featureclass technique.