When the Morph Type field displays –infix-, an Infix Position field appears (entry level). If you insert an affix allomorph that is an infix, another Infix Position field appears below the Allomorphs field. These fields store one or more environments.
In the Navigation Pane, click Lexicon, and then click Lexicon Edit.
In the Entries column, click the desired entry.
In the Entry pane, if you cannot see the Infix Position field, show hidden fields.
Click the Infix Position field, and then click the ellipsis button 
that appears.
The Choose Infix Position dialog box appears.
Do any of these steps:
Click the Edit the Environments link to insert or edit an environment.
Select (
) one or more environments. Click OK.
TipYou can also type an environment directly in the Infix Positions field, if you type the correct syntax or you use the menu commands that appear when you click the menu button 
for the field.
After you enter the environment, leave that field by clicking another field. If a red underline appears below the environment, the environment has a problem. If you cannot determine and solve the problem, refer to the Problems area.
If you type a new environment, you will need to add its name and description it in the Grammar area.
Refer to Environment String Representations for string representation syntax examples.
For more information, point to Resources on the Help menu, and then click Introduction to Parsing.