This is it!

The way I was downloading files was by right click -> save as and it created exe file which obviously was not the actual installer.
You have to click on the link and get to prompt screen with license agreement to download actual Executable.

I would place a small note on download pages ...

I tried this already and the information displayed is 64bit Operating System as you can see in screenshot.

The system runs in x64bit - all the other programs installed are 64bit.
Please see attachment.

It seems that the problem is in how installer detecting the OS not the OS itself.

Trying to install full x64bit version of OctaneRender from installer results in message attached.

Same result for following installers:
OctaneRender_2_25_win64.exe
OctaneRender_2_24_2_win64.exe
OctaneRender_for_DazStudio_2_24_2_5_Win64.exe

The PC I am trying to install on is - Windows 7 64bit ...