Hi!
There is no remmina in repository. It seems to me it was in salix 14. I tried to build it from source but it has too much dependencies.
Remmina in 15.0
Re: Remmina in 15.0
Just added it, along with respective deps in the extra repos for 15.0. Wait until your mirror syncs and you will be able to install it with gslapt/slapt-get.