If you don't have a PC, you can't fully use this MP3 player's features.
Literal
PC [object-を] have-not person [topic-は], this MP3 player [genitive-の] function [object-を] full [target-に] use-master-cannot [polite-ません].
使いこなす ('to master, use fully, handle skillfully') is a compound verb: 使う ('use') + こなす ('digest, master, handle with skill'). The potential negative 使いこなせません means 'can't fully utilize.' フルに is the katakana loanword 'full' + adverbial に. Typical of product documentation or user guides.