Looks like you're using script in an XML-based (.rpx) report instead of a purely code-based report. When using XML-based reports (.rpx files) in older versions of ActiveReports you must access the controls via collections as follows:
((DataDynamics.ActiveReports.TextBox)rpt.Sections["Detail1"].Controls["TextBox1"]).Text = "Hello World";
However, in the current version of ActiveReports (ActiveReports 7) this limitation has been removed so you can write code as follows:
this.TextBox1.Text = "Hello World";