Once you log in and you're on the summary page, click "positions". You should have three options, Overview, Dividend View and Fund Performance. If you click Dividend View, it will give you the dividend yield on anything in the portfolio, including SPAXX. It's currently paying 4.74%.
Edit: The yield for SPAXX is always Your yield, because the shares stay at a fixed $1. If you are looking at dividend yield on any other kind of investment, like a dividend paying stock, be aware that the yield shown is calculated on shares bought at today's market price (i.e., if you bought more shares of XYZ, that's what they'd yield you). It is NOT the yield you are receiving on shares you already own, you'd have to use your own basis to calculate that.