Unfortunately, according to an official announcement made by the company, the application has been discontinued and is no longer available to download from official sources.
What's even worse is that I don't know of any other application that's able to either convert or open the OR6 format, other than Lotus Organizer 6.1. At one point Outlook had this capability, but I'm not sure if the newer versions can do it or not.
In case you still have the installer, you just can't get it to run on Windows 10, here's a link to a Windows patch: http://www-01.ibm.com/support/docview.wss?uid=swg21228070 and to an update http://www-01.ibm.com/support/docview.wss?uid=swg24006178 that may prove of some assistance. Additionally, you could try creating a virtual machine with an older Windows version on it, then installing Lotus Organizer on that machine, but once again this only works if you already have the installer saved on your disk or on a storage device.