I'm not sure what you mean by "private equity (ISOs). If you sold Incentive Stock Options (ISOs) in a disqualifying disposition, the bargain element (the difference between what you paid for the stock and its fair market value on the day you bought it) is considered ordinary income. The income, and the related tax withholding, including FICA should have been reported on your W-2. It is not something you can add to your tax return. You need to contact your employer this.
If you are not an employee, then you may have received non-qualified stock options (NQSOs), as they can be issued to certain non-employees.
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