In order to make a field visible for a requirement you need to add it to the definition for each Type of Requirement in which you want it to appear.
Go to Tools > Customize > Requirement Types.
From the middle pane listing Types of Requirements, select the Type where you want the field included.
The click on the User Defined Fields tab in the right-side pane. This should show all User Defined fields you have set up for Requirements.
Check mark the "In Type" field for each User Defined Field you want included in this Type of Requirement. Optionally you can also indicate if it is Required for this Type of Requirement.
Click Save.
Repeat for each Type of Requirement where you want the field included.
Go to Tools > Customize > Requirement Types.
From the middle pane listing Types of Requirements, select the Type where you want the field included.
The click on the User Defined Fields tab in the right-side pane. This should show all User Defined fields you have set up for Requirements.
Check mark the "In Type" field for each User Defined Field you want included in this Type of Requirement. Optionally you can also indicate if it is Required for this Type of Requirement.
Click Save.
Repeat for each Type of Requirement where you want the field included.