If you are installing the HL7 accelerator on a 64 bit development environment, make sure that you install both BizTalk 2006 R2 and the HL7 components in the \Program Files (x86)\ folder.
If you don't the HL7 projects (BTAHL72X Common, ...) will not show up as available as BizTalk projects. You will still be able to create HL7 schemas using the new button'd dialog box, but the underlying projects are not available.