Hi,
if you have a macro already set up than simply have database window active on the macro's tab and form in design view active in the background. Just click and drag the macro over the form and as the cursor moves to form it will change into the button.
Place the macro/button where you want it and that should be it.
Alternatively create a normal button and under properties 'On Click' select the macro from the drop down box.
HTH
Chris