If I remember correctly, DOS versions of Lotus used expanded memory. My guess is that you have a line in your CONFIG.SYS that is similar to:
DEVICE=C:\DOS\EMM386.EXE NOEMS
So, to give your computer some expanded memory take away the NOEMS parameter on the EMM386.EXE line. If you need MORE expanded RAM, then try adding a number after the line. If your computer has more than 32 MB of RAM, you can try
DEVICE=C:\DOS\EMM386.EXE 32767
or use a smaller number if you have 32 MB or less RAM in your computer.