I just tested the issue you reported, using TurboTax Home and Business for Mac. I found that you can go into Forms mode and enter data in any of the non-computational lines on Form 3510, except Line 28.
I will report this issue for further review. Please post again if you have found anything else not working with Form 3510.
Update: This is an identified issue. Users who wish to sign up to be notified of progress on this issue can do so here.
[Edited 03/31/24| 5:49pm PST]
@matteartist
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