I went to upgrade to Acid Pro 11 and the 64 bit version was installed (because my system is) but it does not work with my 32-bit plug ins (Native Instruments soft synths). How do I install the 32 bit version? I know I have done this for previous versions.
Acid Pro 11 is 64 bit only. See the Operating System section of system requirements here, and this page on the Magix website which explains how to recognise whether a 32 bit version is an option.