The MIDI Manager has been discontinued and is unsupported. The official MIDI interface is now Opcode's OMS (Open Music System). The latest version is 2.3.8 for Mac OS 7.5.5 through Mac OS 7.6.1 users. It is available for downloading from several sites. You may want to try the following site:
http://www.opcode.com/downloads/OMS_2.3.8.hqx .