As best I can tell, statsmodels 0.5.0 simply doesn't work with Python 3.4, even with Cython 0.20.1 (latest) installed. The latest master installed fine, however, so here's one approach if you're willing to use an unreleased version:
git clone https://github.com/statsmodels/statsmodels
cd statsmodels
pip install .
Update: This shouldn't be necessary using the latest version - see Rasmus Larsen's comment below.