Basically the written test will validate that you are ready to take the ACMX practical exam. So make sure you are prepared to take that exam before you do the written ACMX test.
Check in this Airheads forum on ACMX, and in this topic there is some good dicussion on ACMX.
Few higlights from there:
- Start here on this page.
- Make sure you have sufficient practical hands on experience to deploy an advanced Aruba network.
- Take the Advanced Troubleshooting class.
- Check out the ACMX Study Guide, PDF here, more up to date page here.