The difference between missing premise and argument completion drills is that the scope of the drills is different. With missing premise drills, you're filling in a premise. With argument completion drills, you're looking to complete the argument, and maybe you're adding a conclusion or a premise. In other words, argument completion drills generally can encompass more than missing premise drills.
I recommend reviewing the curriculum on quantifiers, as there are many examples there that show you how to solve for the missing premise. The video lectures will also help you to internalize the strategies for solving these questions.
For specific questions, let us know by posting questions to the message board for the questions you're wanting further clarification on so we can help. Let us know if you have any other questions!
