You treat it like any other account. First you enable the plugin. Then you add an account of type SMS (Settings...Configure Kopete...Accounts...New). You'll arrive at the dialogue which allows you to type in an account name and choose your sms app. I installed smssend so that's what I chose. You can find it here - ( http://zekiller.skytech.org/smssend_menu_en.html ). From the drop-down choose smssend, then select the SMSSend prefix directory... in my case it was /usr. You'll know its right if a bunch of providers are listed in the drop-down box. Now I'm a sprintpcs user so that's who I chose from providers. Enter your callback number.
Now add a contact for that account. And send away.