Plugins or Filezilla

Hello, i am very new in this and i need help. I use Fictive theme http://www.zivotjepokret.com and i want to use few plugins but couldnt find that option. I start reading about Filezilla but failed to make child theme because i cant connect to my site. Questions are. Can i use plugins with this theme? And what is my user name and password for Filezilla if they are not my user name and password for wordpress? Thank you in advance for answer.