The option in the admin is to show the other products on the thank you page.
If you want to promote your items in the shopping cart itself, you will need to use JavaScript manipulation for that.
Please add the following your add to cart button code:
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
// XXXX is item number of the product which when added will trigger the upsell,
// YYYY is the item number of the product being promoted in the cart.
if (EJEJC_added('XXXX')!==false && EJEJC_added('YYYY')===false) {
jQuery('#tdSell').attr("innerHTML", '<a
href="https://www.e-junkie.com/ecom/gb.php?c=cart&i=YYYY&cl=...&ejc=2"
target="ej_ejc" class="ec_ejc_thkbx" onClick="javascript:return
EJEJC_lc(this);">Buy Our other product!</a>');
}
}