When you start a new 90-30 project, Proficy will default to a CPU374 which has an Ethernet port. The Ethernet port has 80 %I bits of status information which Proficy defaults to start at %I00001. Thus, the next available %I address for the MDL645 is %I00081. You can change the starting address for the Ethernet port status bits to free up the address you want for the MDL645.
Remember also that to download a new hardware configuration the PLC must be stopped. When you begin a download with the PLC stopped, you get a choice of downloading the program, the configuration and the initial values. If you begin a download while the PLC is running you don't get those choices. All you download is the program.