Hi areginato and welcome to the Q&A Board! So glad you're enjoying digital photography so much.
I can't find much info on the Hanimex TX36 flash unit. Is it a dedicated slave flash? If not, but it can be used as a slave flash, there is usually a button to switch it into slave mode. It should go off when the Kodak built-in flash is triggered.
Remember, both flashes will go off. If your camera has flash output setttings, to increase or decrease the strength of the flash, try changing that for improved lighting of a subject.
Also check the camera manual for any special instructions about how to use it with a slave flash.
If you give us some more insights about your use of it, perhaps we can give more specific help.
Related reading - slave flashes
http://www.digicamhelp.com/accessories/external-flashes/slave-flash.php