In 5.5 there's a field tool in the left hand status area that you use to add a field to your layout. This tool is located at the top of the screen in the now horizontal status area. Drag and drop from that tool to where you want to add your field. This opens up the specify field dialog. Select the desired table occurrence from which you want to select a field in the drop down at the top of this dialog. Click a field to select it and click the check box for the field name if you want it added. Now click OK.
Table Occurrences will be a new concept for you. Every table you create will get a table occurrence "box" in manage relationships of the same name. These "boxes" are what you use to define relationships between the different tables in your file. Thus, when adding a field to your layout, you must indicate from which table occurrence you are selcting it to tell FileMaker what relationship to use to determine what record and in what table the value shown on your layout will be stored.