Two-tier femtocell networks, which comprise macro-cells underlaid with short range femtocells, are under intensive study due to their improved indoor coverage and data rate advantages as compared to conventional single-tier macrocellular networks. Nevertheless, the implementation of two-tier networks with universal frequency reuse causes severe cross-tier interference from the femtocells to the macrocellular users, exacerbated by dense femtocell deployment. In this paper, we investigate the effect of access control and cell association in two-tier networks, where the macrocells employ closed access, whereas the femtocells can operate in either open or closed access. By introducing a tractable model, we derive the success probability for each tier under different femtocell access schemes using stochastic geometric tools. For the case of open access, we derive the enhanced success probability for a macrocell user, and we show that the effect of inter-tier interference could be eliminated through optimal cell association policy for the open access.