I just discovered there is a bug in the RC version of VS 2010 it is not generating the designer code for controls within an Ajax Tab Control. Have a look here
Will generate the following line in the designer code file protected global::System.Web.UI.WebControls.TextBox txJoe;
However in VS2010 this does not get generated and so you cannot access the control in your code behind file. Hopefully we will get a fix soon and don’t have to wait for the RTM.