For anyone who is still working on this thread:

I am running the latest version of Mac OSX (10.4.8) and I just got my old version of Lotus 1-2-3 (1.10.00) to run on my powerbook. All I had to do was put 500000 KB (yes you read it right) in the memory drop down menu in the GET INFO dialog (you can access this by clicking once on the 1-2-3 application icon with command I or going in the finder's File drop down menu). My Lotus has returned!

Hope you folks are still keeping an eye on this thread.

I think your problem is the amount of ram available. I have found Lotus 123 will not launch if there is too much ram !!! weird ..hence the reason it will launch when other app are running.

Lotus 1-2-3 for the Mac was not a 32-bit clean application, meaning that it used some of the high 8 bits of memory pointers for its own purposes. At the time, no Macs more than 64 MB of RAM, so the program could get away with it. As a safety measure, the program would check to see that it was runnning in the first 128MB on the system (so that none of the highest bits would be set by just landing in memory), and quit immediately. Starting in Mac OS 9.2-ish, the Mac OS started loading applications high in memory. That, combined with systems having lots of RAM, made 1-2-3 for the Mac detect the pointer problem and quit.

It is possible to still run even on 10.4's Classic environment. The trick is to simply run another classic app (e.g. SimpleText), with its memory partition set to something enormous, like 256MB. This will cause SimpleText to occupy the "high" memory region, 1-2-3 will load low, and its address sniffer will be placated.

At last I'm getting closer.
Forgive me for going a bit off topic, but this is the first anything I've found which comes even close to what I'm seeking. . I'm a brand new Mac user (MacBook Pro 17) but a very old Lotus 123 user since 1986, currently on a Windows XP Pro desktop PC. I have long known about the wonders of Mac, but always constrained to PC by employment. I'm retired and my own boss, with long term 123 experience and the last 7 years of self employed stuff in 123. So I really would like to have 123 on my MacBook.
I'm totally ignorant and struggling with Mac, but I think it's a glorious machine. I had a try with "Crossover" today, but it wouldn't install 123 or Microsoft Autoroute, both of which on the MacBook would make my life complete. I've heard of Parallels and Bootcamp. Can anybody set me in the right direction to get productive, please? Please remember I'm a Mac ignoramus - I've already got the desktop looking like a disaster area simply because I don't know how to look after it - my PC knowledge is no help.

Where did the number 500000 come from? Is is in any way related to the RAM on your system? If I use a different number what might be the result?