We investigate the use of natural language to drive the generalization of control policies and introduce the new multi-task environment MESSENGER with free-form text manuals describing the environment dynamics. Unlike previous work, MESSENGER does not assume prior knowledge connecting text and state observations - the control policy must simultaneously ground the game manual to entity symbols and dynamics in the environment. We develop a new model, EMMA (Entity Mapper with Multi-modal Attention) which uses an entity-conditioned attention module that allows for selective focus over relevant descriptions in the manual for each entity in the environment. EMMA is end-to-end differentiable and learns a latent grounding of entities and dynamics from text to observations using only environment rewards. EMMA achieves successful zero-shot generalization to unseen games with new dynamics, obtaining a 40% higher win rate compared to multiple baselines. However, win rate on the hardest stage of MESSENGER remains low (10%), demonstrating the need for additional work in this direction.