First, make absolutely certain that your chain is long enough to engage the big chainring (in front) and the big cog(rear) without straining the derailleur. Do this by turning the crank GENTLY as you shift and STOP if there is significant resistance.
If your chain was near the minimum length required for the 25T cog it will be too short for the 30T and can cause extensive damage if you accidentally shift into that combination while pedaling.
Assuming that your rear derailleur will handle the 30T cog (not guaranteed that it will) you should be able to adjust the derailleur using this procedure:
http://www.parktool.com/blog/repair-...nts-derailleur
Make certain that there are not any unneeded spacers (perhaps left over from the old cassette) under the cassette before you begin; if present they will push the cassette away from the hub and possibly out of reach of the derailleur.