Hm, I think you lack a very basic understanding of the techniques around remote desktop protocols, plugins, etc.
1) remmina is a graphical user interface to create connections to other pcs/servers. You click on "New" and create a connection.
2) remmina can be extended by remmina-plugins, which supports MS RDP, Nomachine NX, VNC and other protocols.
3) These plugins each might need some special programs additionally. E.g. MS RDP needs freerdp, NX needs nxproxy/nxcomp, etc.
freerdp doesn't deal with NX stuff at all. It's just installed as a dep of the RDP plugin. What you need is nxproxy and nxcomp. A few minutes ago there just was no nxproxy you could use in tinycore because there was no extension providing it. But now I did that and nxproxy is installed as a dep of remmina-plugins. So what's left for you to do?
Open a terminal and run
$ tce-update
...and try remmina again.
[edit]
You might have to install the nxproxy and shared-mime-info extension manually when updating since they are new.
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