What if I don't know where my parents applied for my green card?(I-90)

Don't delay the process. Fill out your form completely.

The USCIS requires that you complete the form completely and accurately. If you do not know, or are unable to locate any information and choose to leave it blank, the USCIS may contact you and request the missing information, which could delay the process. CITATION: http://www.uscis.gov/forms-filing-tips