Thank you for your post.
For information to fill in, FileMaker needs to look up the information somewhere. Somewhere, you probably have a table that holds the Unit, Airfield Identifier, Phone Number, and probably several other fields.
Therefore, you need to create a relationship between that table and the table where you want to input the Unit information.
Pull down the File menu and select "Manage -> Database". Select the Relationships tab, and you should see the tables. Find the Unit field in one table, click and drag it over to the Unit field in the other table. When you let go, there will now be a line between the two fields, and the fields will be placed at the top of each table. Double-click the icon between the two tables, and select the option "Allow creation of records in this table via this relationship" on both sides. Click OK and return to your database.
Pull down the View menu and select "Layout Mode". On the left side of the screen, you will see several tools. Click on the tool just below the oval tool. This is the portal tool, which allows you to draw an area on the layout to view information from related tables. Select the Airfield Identifier and Phone Number from the related table, and select the Unit from the current table. Click OK, and return to Browse mode.
Now, go to the portal, enter a Unit Number, and the Airfield Identifier and Phone Number associated with that Unit appears.
If you need clarification for any of the above steps, please let me know.