the syntax should be something like this -- it involves casting, and the type you cast to depends on the ChartType of your ChartLayer... in this case i'm using a ColumnChartAppearance.

ChartTextAppearance myChartText = new ChartTextAppearance();
myChartText.Column = myChartText.Row = -2; // wildcard
myChartText.Visible = true;

ColumnChartAppearance columnApp = this.ultraChart1.CompositeChart.ChartLayers[0].ChartTypeAppearance as ColumnChartAppearance;
columnApp.ChartText.Add(myChartText);