The short answer here is that you should be able to, but plan mode currently doesn’t not support that functionality due to a artifact of its development history. So we are in the process of changing that. Our plan is to more tightly integrate Plan and Actuals mode so that they look the same and behave the same way (like Actuals does now). But when you are in Plan mode you will be able to edit the plan values in the upper left of cells, and in Actuals mode you edit the actual values in the lower right. In addition, you will be able to assign resources in either mode.
For now, you just have to write your planned resources to the Actuals tab (where you can also view, but not edit the plan values).