Motion blur is not directly available for Nuke 3d geo items. So to get motion blur on a Sphere item you need to export it to an OCS and then import it as a reference item, as foillows:
1) Add a Write node to the Nuke node graph and plug the OctaneRender node into it. Then set the "file" to temp.ocs and the "file type" to OCS. Click Render to output the sphere to the OCS file.
2) Add an OctaneReferenceGeometry3d node, and set the ORBX/OCS to the file output from 1). Plug this node into the OctaneRender node, and unplug the sphere from the Scene node.
3) Start rendering, and set the Shutter Speed to the applicable number of milliseconds (the "Shutter Speed (secs)" label should read "Shutter Speed (msecs)", which I will fix in the next release.
When I tried step 1) with the sample scene you sent me, I got an octane error related to the octane scene version. However when I created a new scene with the latest version of the plugin (which I will post in the Releases forum as a pre-release later today), I did not have this problem - so use the later version for this process.