you cannot do an online migration with MS Access as you can do with other foreign datasources. You can only do an offline migration.
The first step that you need to do is to call the Microsoft Access Exporter. Click on Tools - Migration - Microsoft Access Exporter and chose the exporter of the MS Access version that you are using.
The exporter creates an xml file for you. After the exporter has finished you can do the migration, click on Tools - Migration - Migrate, but in Step 4 please select "Offline", and then you can select the xml file that the exporter has created. The following steps in the Migration assistant are then doing the migration for you.
You might also want to read the chapter "Before Migrating From Microsoft Access" in the online help of SQL Developer.