How to install the package syms into GNU Octave, ver. 5.2.0?

Dear colleagues,

I would like to install the package syms into Octave. I have downloaded 
and unpacked symbolic-2.9.0.tar.gz in d:\installs\Octave\. Command

pkg install -forge d:\installs\Octave\symbolic-2.9.0\inst\syms.m

reports:

error: get_forge_pkg: invalid package NAME: d:\installs\Octave\symbolic-2.9.0\inst\syms.m
error: called from
    get_forge_pkg at line 34 column 5
    get_forge_download at line 26 column 14
    pkg at line 407 column 31

Why is package name invalid? Whre is a problem?

Thanks for your answer.

Dalibor Martišek