Sum[1/i^2, {i, 1, Infinity}]
Or you can the Palettes menu at the top of the page. Click Basic Math Assistant... then click the Sum button in the palette. Then fill in the appropriate fields and press shift enter.
By the way... the "sum from 1 to infinity of n+2" does not coverage. Try
Sum[i + 2, {i, 1, Infinity}]
