To track an I bond, you can set up an off-line Brokerage type investing account to hold all your I bonds. You can call it "Treasury Direct" or whatever you like.
Then go to Tools > Security List and click on Add security to create a Security for the bond. Click on the link at the bottom to enter the name manually. Leave the Ticker Symbol blank and give the bond a descriptive name like IBond 2052-08-31. Set the Security type to Other. The Bond type is for negotiable bonds, which does not apply to I bonds.
Usually people defer I bond interest until they redeem the bond. The bond's value is increasing year by year and thus should be reflected in your net worth, but the increase should not show as taxable income.
In this case, go to the account you created and enter a Buy (not Bonds bought) for one “share” at the amount you paid for the bond. If you receive the bond as a gift, you would enter an Add instead.
As interest accrues and the I bond’s value increases over the years, you can increase the bond's "share" price to match its current value. Then when you redeem the bond, you would [edit per @q_lurker's comments below] lower the price back to the purchase price sell the bond at the original purchase price and record an Int payment for the total interest. then record the sale . That should make Quicken's net worth and tax calculations correct.
If you choose to pay taxes as you go, you would buy as many shares of the security you created as the amount you paid for the bond at a price of $1.00. When the bond pays interest, you record it as a ReinvInt transaction.