What code are you using today to access the textbox? In general, accessing a control in a RadGrid cell looks something like this:
var myTxtBox = myGridDataItem["columnName"].Controls as TextBox;
var strText = myTxtBox.Text;
You access the control in the cell that you're targeting, cast it to the correct object type, and then you should have full access to the properties. If you want access to databound values, you need to do this in the OnItemDataBound (or later) event. More details here:
Hope that helps.