There's a section in the H9 Control user manual that deals with connectivity issues. Just look under the section "Connecting to Devices":
http://eventide.com/AudioDivision/Products/StompBoxes/H9ControlUG1.aspx
This manual is also bundled into H9 Control under Settings / Help.
If the user manual doesn't help, send an email to support describing what operating system you're on and whether the H9 appears under the Device Manager on Windows or Audio MIDI Setup on Mac.
