To Add a Line Item on the ProDeal iOS app...
Go to the Checklist room view and click the three dot menu button on the right side of a Heading .
This will open up a pop-up menu. Click Add Line Items.

Type in the title of your line item and click Add. To add multiple Line Items at once, type in another title and click Add. Then click Save.
You can easily delete existing Line Items from this screen by clicking the "-" icon.

Tip: You can add Subfolders the same way.