There's a number of reasons as to why you are not receiving the label download email.
1) There's a slight delay in generating the email - should this be the case, simply log into your account and click ‘My Account’. Under the ‘Request Shipping Label(s)’ button for your chosen program, you should be able to see a ‘See Shipment History’ link. Once you click this, you will be able to see all shipments for this program including any label requests. This will let you download your label.
2) Your account details are incorrect - to check your account details, log into your TerraCycle account and click 'My Personal Info' and double-check that your details are filled into the corresponding fields especially City, Region and Postal Code. If they are not, you will need to amend your details by clicking 'Edit my Info' after which you will need to request a new label.