If you DO NOT administer an etpub server, don't worry about it. This client mod is not intended to be installed by end-users.
If you DO administer an etpub server, then simply copy the etpub_client .pk3 file to your fs_game directory (usually named 'etpub') then restart your server. Note that this requires the client to download the pk3 when they connect so if you have never offered downloads before, now is the time to learn about ET's download settings.
etpub_client-YYYYMMDD.pk3
The date is used instead of the version of etpub it was built for a couple of reasons.
First, each release of etpub will not require a new etpub_client. For example, if there was an etpub_client-20050516.pk3, you would be able to use it for etpub versions 0.5.1, 0.5.2, and 0.6.1.
Second, the etpub_client could be updated between etpub releases so if you're running etpub_client-20050516.pk3 with etpub 0.5.1, etpub_client may be updated and you would run etpub_client-20050517.pk3 with etpub 0.5.1.
Why the date instead of a seperate versioning system? Most of our work is spent on etpub. etpub_client is more of a companion to that work than a seperate project. Therefore we don't need a versioning scheme to aid in development. Adding a seperate versioning scheme would also confuse admins about compatibility between etpub and etpub_client.