An investment account is the cash account (usually a money market account) that you hold at your brokerage. This is where your brokerage holds the funds you use to make stock, mutual fund and other investment purchases and where the brokerage deposits the proceeds from the sales you execute. 

To add: 

Select Transfer & Pay, External Funds Transfer, Setting, then select Add a New Account tab. You'll be brought to the Add Accounts screen. Select Brokerage from the drop down box and choose your Brokerage Account name. Then add your account information and click Add. 

To modify or delete, select account and then Edit or Delete Account.

Depending on your particular brokerage, you may be asked whether or not you have check-writing capabilities for your account. If your account includes check-writing privileges, you'll be asked for the ABA routing number that appears on your check.

When you have supplied all requested information, click the button labeled Add. You will be presented with the page titled Overview.