I am presenting at my first international science conference. It is a bit daunting but I feel very proud of my work. However, some seniors don't seem that impressed and are only interested in publications. How important are presentations at national/international conferences?

from my experience, presentations can be important, if not just for the practice, but also if a presentation goes into a proceedings book, for that conference you can call it a publication and can go toward a RAE rating for your department. Anything of this kind will improve your CV.

use any feedback you get at the conference to fine tune your paper, write it up and try getting it published - this should keep the head of department happy! Good luck