I wish I could give you a reliable description of what is and isn't going on, but discovery is really unreliable for me, and a lack of responsiveness and UI feedback keeps me from figuring it all out.
I have two M1001s and an R1000, all running 2.7.78. The first time I launched the program, I got the dialog asking whether I wanted to specify or discover units. I clicked Discover, and nothing happened. (Not even a progress indicator. Can I suggest a UI element for searching the network?) I had no idea whether it had crashed, failed to find any of the three SBs, or was still searching for them.
There's no apparently way to continue when that happens, so I quit SBC and relaunched it. This time, it quickly found all three SBs. I selected all three and clicked Select Device, and one opened. But it didn't tell me which one of the three it was. I have them named uniquely, but while those unique names were displayed in the Select Device dialog, neither the name nor the IP address was in the device's window. I finally found SoundBridge Info in the Tools menu, but it didn't give me the name, just the IP address. Fortunately, since I assign static IPs to them, I could take an informed guess at which one it was. (I assigned those IPs a couple of years ago but fortunately assigned from the back to the front of my apartment.)
I wanted to see the others, so I went to the Discovery menu and chose Add a SoundBridge and Discover one. This time it took about eight minutes to find all three units -- the last one being the one that was already open. (They were found individually a little less than three minutes apart.) I selected the R1000 this time, but it didn't open. The M1001's window seemed to refresh, and I thought perhaps multiple windows were disallowed, but checking Tools/Soundbridge Info, it's still displaying the original M1001. Ironically, the status window shows me an icon I don't recognize on the left, a tiny M1000 on the right, and a ton of white space in the middle.
It's a full five minutes later and the R1000 still hasn't opened.
The info I can give you is that I'm using a Macbook Pro from mid-2006, running 10.4.11, with a Netgear B/G router and nothing else currently active on the network. As I say, all three Soundbridges are at 2.7.78.
If there's anything else I can do to help you troubleshoot, please let me know, and if you need more detailed knowledge about my system or network, let me know via PM.
I do have to admit that while features like the artist list look very useful, I do prefer the more compact size of the earlier SBC. (And I know this version is customizable, but with three SBs here, I can't see how to display all three as compactly as the earlier version did. I do look forward to playing with it, but it isn't a priority since the old one works so well and this is currently so slow.)