Std search button should work - see the binocular icon above your buttons - 4th from left. You can enter the string and if it find it will display it.
If that doesn't work for you - you can create your own popup to ask for the value using the FM POPUP_GET_VALUES. Once you capture the value and search within your internal table, you can use the method SET_SELECTED_CELLS_ID of CL_GUI_ALV_GRID to set the cursor on the cell.
Thanks,
Naimesh Patel