We study the freezing of CCl4 in microporous activated carbon fibers (ACF), using Monte Carlo simulation and differential scanning calorimetry (DSC). Microporous activated carbon fibers are well characterized porous materials, having slit-shaped pores due to the voids formed between graphitic basal planes. They serve as highly attractive adsorbents for simple nonpolar molecules, the adsorbent-adsorbate interaction being mostly dispersive (of the van der Waals-type). Recent molecular simulation studies have predicted an upward shift in the freezing temperature (Delta T-f=T-f,T-pore-T-f,T-bulk> 0) for simple fluids confined in such highly attractive carbon slit pores. Our DSC experiments verify these predictions about the increase in T-f. The results also indicate significant deviation from the prediction of Delta T-f based on the Gibbs-Thomson equation (simple capillary theory). We employ a recently developed free energy method to calculate the exact freezing temperature in these confined systems using molecular simulation, in order to address the failure of the simple capillary theory.