MSFlexGrid and MSHFlexGrid controls are both read-only. So you can use TextBox in the following way:
1. Put TextBox over the grid. Move and resize TextBox to position them exactly over cell of the grid, so user will not see that is separate textbox: it will think that is textbox from the grid.
2. When you must get or write data from/to TextBox, use this row and field from the recordset, which corresponds to the cell, over which is TextBox.
3. When user moves from one cell to another (by the mouse or keyboard arrows), move TextBox over new cell and fill them with data from appropriate record and field.

In this way you can put also ComboBox over the grid and give user ability to select from the combobox. In this case you must maintain separate recordset behind combobox and repopulate them on every move over different cell with appropriate choices for that cell.