Amended tax returns?? Federal and State?

On our 2009 federal tax return, we used the First time home-buyers credit but forgot to add in the Schedule M, Making Work Pay Credit, and the IRS did NOT send it to us after the fact like they did with some others. Can we file an ameneded return and if so what paperwork do we need? We have a similar problem with our MO 2009, in that we didn't put our federal taxes paid on there (one of 6 states that allows this deduction!). It made a big enough difference this year so we were going to amend last year. What paperwork do I need? Does amending your taxes leave you open for audits?