I assume you want to add a button to the grid (cell) and process an existing macro when you click the button (and you use Office version different from "Office 2007"). Moreover the existing macro looks like this:
Private Sub Hello_World() MsgBox "Hello World"End Sub
1) Right-mouse click on the toolbar area -> list of toolbars opens -> check "Control Toolbox" (On the top, a toogle button is displayed to switch design mode on/off).
2) Press the "Command Button" icon
3) Add a button to the area/cell of the grid
4) Doubleclick the button
-> Visual basic displays:
Private Sub CommandButton1_Click()
5) Add your existing macro to the sub "CommandButton1_Click"
Private Sub CommandButton1_Click() Hello_WorldEnd Sub
6) Switch Design mode off
7) Press button -> Pop-Up is displayed with conteent "Hello World"