Are you root when you run the dpkg command?
Let me explain a bit. Apt-get is not the package tool. It is more of a package management system which takes care of indexing and dependencies etc. Many make the mistake of comparing RPM to apt-get which is totally bogus. They should really be comparing RPM to dpkg. The dpkg tool is the tool that apt-get uses to install packages (that have a .deb ending similar to .rpm ending for RPM files). This explains why Fedora, SuSE etc can use apt-get with RPM's.
The above is true for Debian in any case and all Debian derived distros that I have worked with.
Yoper on the other hand promises to be able to combine installations of rpm's and deb packages. Why is this strange? Well, because both packaging systems use their own databases to take care of dependencies and indexing what is available, what is installed and so on.
So how is Yoper accomplishing this? I don't know but my guess is that they are probably using either system and not both with some fix that can make them coexist peacefully on the same system. If they had accomplished such a fix then I would assume that all distros would be running it.
My best guess is that Yoper uses apt-get for RPM's. When you want to install a debian package, they just use a tool such as "alien" to change it from .deb to .rpm. I am not sure of this but I would think that they are doing something of the sort. This could be the reason why it says it can't find dpkg - because it does not exist and is not used under Yoper.
So what I would recommend is that you join the Yoper forums at www.yoper.com
and ask around (or search) how to get sensors up and running. I can only assume that the install process is specific to Yoper.
I am sorry I can't be of further help here but I simply don't know Yoper well enough to comment on its particulars.