Hi.
The solution silence has posted above, works just fine.
But just a little remark for those who are new to Linux and/or N900 like me:
A manually install means to me to use the apt-get command.

So here is how I did it:

* Be sure that the extras-devel repository is deactivated
* install the roothsh
* open the X Terminal and gain root with sudo gainroot
* type in apt-get install avahi-deamon
* open the program manager and activate the extras-devel
* install pc-connectivity...

and hopefully enjoy a powerful tool ;-)

Thanks to silence and all the other helpful postings!