Hicom 300E is typically programmed in one of 3 ways. One, you need an application called LC-Win, and you must use the version of the application that matches the software release of your system (6.4, 6.5 or 6.6). You will need to know the customer or service login and password. Second, is you must have ProComm Plus for Windows, and you must have the 3.5" floppy disk that comes with the system tapes for the PBX. On there are three keyboard definition files you must copy into your procomm directory: pansi.kbd pansi3.kbd and tvi910.kbd. You would connect your computer running Procomm to the open access port on the Custom Callout Adapter using a normal serial cable. Set up Procomm for a new direct connection, using ANSI-BBS terminal emulation, and edit the perminal emulation settings so it yses the pansi3.kbd file you copied on. The connection speed varies from 19.200 baud for 9006.4 to 115,200 for 9006.6, and depends on if you have a CBX Monitor III (19,200), Custom Callout Adapter (38,400), or Custom Callout Adapter II (115,200) - all of them for access through the CCA use 8 - N - 1 for the protocol. You can also program procomm to use dial-up and call into the phone number that is connected to the CCA for alarm dialing purposes. Once you get connected and get a login prompt you will need the customer or service login and password. The 3rd way is also using Procomm, and connecting to the RMX port using a null modem, TVI910 emulation and 4800 E-7-1 for the comm settings. That is much more difficult so I won't go into it more than that.
Once you get logged in, if you have an older version like 9006.3 or 9006.4 you can use the EMML dialog to do some of the basic programming, the first version of 9006.5 lets you do a little that way, and the second version of 9006.5 and 9006.6 require you to use LC-Win.
Hope that helps. Programming the system is not as simple as the little key systems where you can program everything off a master phone, or the Hicom 150s and maybe some of the 3000 models.