Can I offset capital gains with margin interest? if so, how do I do it? I'm completing my return online and I don't seem to be given the option.

No, you can't offset capital gains by margin interest.

Margin interest is investment interest expense.  Deductible on Sch A.  So you need to itemize deductions.  Investment interest expense also has limitations on the amount. (limited to NET investment income)

See Publication 550

https://www.irs.gov/publications/p550/ch03.html#en_US_2016_publink100010207