Actually flip that around (as if it wasn't confusing enough). You have to create your conventional P25 frequency sets first, then create the systems that refer to them. If you create the system without having a frequency set to point it to, the software will cry foul and not let you out of the system creation window without either selecting a frequency set or cancelling the whole operation.
If you plan to use P25 channels you also have to create a P25 group set for the P25 conventional system to refer to. Talkgroup 1 is the standard talkgroup for 700 MHz interop channels in the NIFOG, so I just create a group set called "P25CONV" with Talkgroup 1 in it, and leave it at that. Make it a Global set and I can use it for public safety radios at work, too.
1. Create frequency set(s)
2. Create P25CONV group set
3. Create system(s) to go with frequency set(s)
4. Create zone(s) where you enter the desired system and channel.
One other hint... you have to create a channel name for the software to refer to, up to 8 characters; make the channel name something unique but easy to understand. Once you have a channel name you can add a long channel name (up to 14 or 16 characters, I forget which) that actually resembles English.
And if you're not confused enough already once you've got all that done, you can get into the Options section, where there's all kinds of fun things to tinker with.
My biggest gripe about RPM is that there's no way to import lists of frequencies or talkgroups like there is with RPM2 (at least, not one that I've ever been able to find). Manually entering hundreds of radio channels is an exercise in tedium.
One other fun hint... if there are people on a particular repeater who you want to annoy, enable MDC or G-STAR on that particular channel (the SIGNALING button). The ID blip the radio sends on every transmission will drive your enemies bananas.
I had the same issue. The only way around it was to force the radio to always start up on a particular zone and channel (I used Zone 1, Channel 1 since that's Dispatch where most users are 99.9% of the time); also don't forget to set the System Up/Down buttons to Zone Up/Down.