There's an option for importing matching records. Select this option. Select Donor ID as your match field and then select the check box in the lower left corner of the dialog box that specifies adding a new record if the value in the match field doesn't match.
Note: Be sure to show all records before importing as the import process matches records against your found set, not all records in the table.
Ps. I can see that your table 2 structure is set up to mirror the format of your excel table, but on the FileMaker side of things, putting each month's donation in a separate record instead of a separate field produces a structure that is much more flexible to work with. It is possible to set up an import process that does this, but it also requires a bit more setting up, either you import once for each month's column of data or you import into an extra table and parse the data from that table into individual records via a looping script.