I haven't done this myself, but looking at the site:
1. log into your account
2. click on "Agregar Fondos"
3. Enter an amount under "Depósito en efectivo con Códigos de Barras"
4. Click Aceptar
5. Print out the next page with the bar code on it
You now take that to your RapiPago office along with the cash. They will scan the bar code and deposit that amount to your DineroMail account. They say it takes 2 days.
Hope that helps.