You can transfer domains to your Instra account from another registrar by using the Transfer Domains tool, located in your Instra account at https://instra.secure-admin.com/domain_management/transfer_domain.php
If the domain makes use of a AUTH / EPP code, please ensure the code is correct and up to date, otherwise some delays may incur.
If you are stuck, feel free to contact us over our livechat or by sending an email to support@instra.com