The Store operator is the way to go. You can use it with a combination of Retrieve and Apply Model in another process. If you still have problems please let us know and supply what exactly does not work.
I have found the problem and I think it is good that I report it.
I am working on a private data on a machine that cannot have access to the internet. I have installed RM but I cannot install the extensions, therefore, I have installed the extensions manually by copying the jar file into the plugins directory. Storing and retrieving Weka models did not work. Later, I have copied the extensions into the plugins/managed directory with the XMLfile and I was able to retrieve Weka models by the Retrieve component
Hm, that's strange. Which plugins directory did you use? To make it work you have to copy the extensions to C:\Program Files\Rapid-I\RapidMiner5\lib\plugins (i.e. in the installation directory, C:\Program Files\RapidMiner\RapidMiner5\lib\plugins for the latest version), not to the .RapidMiner5\plugins folder in your home directory. Did you do that?