A system and method for selectively actuating sliding sleeves in valve sub-members which are placed downhole in a wellbore, via one or more darts inserted into said wellbore, to thereby open a ports or ports in such sub-members to allow fracking of the wellbore. The dart is preferably coupled, or provided with coupling means to permit coupling, to a retrieval tool, which upon the retrieval tool being so coupled allows a bypass valve to be opened to thereby assist in withdrawing the dart from within the valve sub-members. Upward movement of the retrieval tool allows a wedge-shaped member to disengage the dart member from a corresponding actuated sleeve to allow the dart to be withdrawn from the wellbore.