Are you using a recent enough subversion of IC5141 (check the date - I think it would need to be from December 2008 or so onwards)?
Do you have the tools link in your MMSIM71 installation - look at <MMSIM71instDir> and see if there is a link called "tools" pointing to tools.lnx86 or tools.sun4v - or whatever platform you're using. I think that must be there, otherwise the flex balance choice wouldn't appear on the form.
Make sure you haven't got another MMSIM release earlier in your path.
Probably the first is the most likely explanation though...