Lorenzo,
What DataGrid control are you using? If it is a grid control that is supported by TestComplete (for instance, Microsoft WPF DataGrid), try using the wValue(Row, Column) property that TestComplete provides for these controls.
If you are using some other grid control that is not supported by TestComplete, then you have to use the grid's native methods and properties to scroll the grid's contents to the needed row and extract the row contents.
To determine which methods and properties to use, see the grid control documentation or talk to your developers.
TestComplete provides access to native methods and properties of WPF objects. To view which methods and properties are available for it, launch your application and explore it in the Object Browser panel.