Hi Ximena,
As a general rule of thumb, the Change Management Plan is the HOWs and the Change Log contains the WHATs .
In other words, the management plans act as a guidance for your project. For example if you are not sure how a change request should be submitted, processed and approved, you can consult the Change Management Plan. If you are not sure what change request have been submitted/approved, you can consult the Change Log for a list of change request items and their statuses.
Hope this helps,
Joe