Symptoms
Upsell products do not display on the cart page.
Cause
The store is not configured properly or the products do not meet the necessary criteria.
Solution
Make sure that the ShowUpsellProductsOnCartPage AppConfig is set to true. Also, upsell products must meet the following criteria or they will not display:
a) Cannot have sizes
b) Cannot have colors
c) Cannot require a text option
d) Cannot be kits
e) Cannot be packs
f) Must be published
g) Cannot be deleted
h) Cannot already be in the shopping cart
If your upsell products have any of those, they should really be used as "related products"