Although this might, at first glance, seem contradictory, it does in fact make great sense.
The point Eisenhower made so eloquently is that although plans invariably need changing during implementation, and so are useless if you stick rigidly to them, the act of planning is what leads to success.
By planning, you get to properly understand the issues and potential hurdles, and you then put a plan together based on this knowledge. As the plan unfolds, however, your knowledge grows, circumstances often change but now the understanding you gained during the planning process enables you adapt the plan to reach your goals successfully.
Without planning, you will have little understanding of how to achieve your goals. With planning you develop understanding and agility and considerably raise the likelihood of success.
So, plan properly, but always remain flexible when implementing your plans.