Does your column is binded to a datasource? If it is not, you should bind in order to show your CFL

Sooorry, I thought you couldn`t open the CFL.

Yes, you need to treat the ChooseFromListAfter event.

There is a sample on SDK that should help you.

C:\Program Files (x86)\SAP\SAP Business One SDK\Samples\COM UI\CSharp\17.ChooseFromList

Seems like I was not treating the ChooseFromListAfter event. I found a blog which was helpful:

http://projectservercode.com/add-cfl-into-the-matrix-columns/

Now I'm having the following error

Cannot cast 'SAPbouiCOM.SBOItemEventArgClass' to 'SAPbouiCOM.ChooseFromListEvent'.

My code is as follow:

        private void Matrix0_CFLA(object sboObject, SAPbouiCOM.SBOItemEventArg pVal)
        {
            try
	        {
                //It Contains information about the current choose from list event,
		        SAPbouiCOM.ChooseFromListEvent oCflEvent = null;
		 
		        //Here pVal is an ItemEvent ,Thus we need to cast it as ChooseFromListEvent to access it as ChooseFromListEvent .
		        oCflEvent = (SAPbouiCOM.ChooseFromListEvent)pVal; // HERE'S THE PROBLEM
		        if (oCflEvent.SelectedObjects != null)
                    ((SAPbouiCOM.EditText)Matrix0.GetCellSpecific(pVal.ColUID, pVal.Row)).Value = oCflEvent.SelectedObjects.GetValue("ItemCode", 0).ToString();
	        }
	        catch(Exception ex)
	        {
                throw ex;
	        }
        }

I believe the problem is the argument you are using "SAPbouiCOM.SBOItemEventArg pVal"

Try to use SAPbouiCOM.ItemEvent.