From OpenNMS

It seems to me that the instructions to install JICMP are too early. The current installer includes 32-bit JICMP, and if you install 64-bit JICMP at step 2, it will be overwritten by the 32-bit files.

I found I needed to overwrite the included 32-bit files with 64-bit versions between steps 8 and 9 of the OpenNMS installation wizard.