I think you should make the second residue in the peptide "A", and apply a modification with the chemical formula "CH2" so that it ends up being transformed into Aib.
From what I read online, the peptide has the sequence:
HAEGTFTSDVSSYLEGQAAKEFIAWLVRGRG
and it has the chemical formula C187H291N45O59, so I think you need to put the CH2 on the first "A" and then put a modification of C35H61N3O12 on the "K".
If you want to see the chemical formula of a peptide, you can create a Report in the Document Grid which includes the "Molecule Formula" column.
If you would like to learn more about Custom Reports and the Document Grid you should look at the Custom Reports tutorial:
https://skyline.ms/wiki/home/software/Skyline/page.view?name=tutorial_custom_reports
If you would like to learn more about working with modifications in Skyline this is the webinar that we recommend:
http://skyline.ms/webinar10.url
-- Nick