There's a few things amiss there; I'll provide a fully corrected version after some explanation. Presumably you're using "ejejc_jQuery" statements (rather than simply "jQuery") because your page is already using a newer version of jQuery. In case you've only been going by other examples from the forum, this help page documents our cart customization code in detail:
http://www.e-junkie.com/ej/help.custom-cart.htm
First, you need to call function EJEJC_config() {EJEJC_POSTCALL=true;} before the function EJEJC_shown(){} -- maybe your page already has this, and you merely omitted it from your example.
Second, inserting form-based Add to Cart code within the cart will not work, because the cart is already itself a form, and forms cannot be nested in HTML. You can insert basic Add to Cart button code and enable "Let buyer edit quantity" in the product's settings instead.
Finally, you need to escape every "&" in the HTML you're adding by adding a preceding backslash, like so:
function EJEJC_config() {
EJEJC_POSTCALL=true;
}
function EJEJC_shown() {
ejejc_jQuery("#imgHeader").attr("src","logo.png");
ejejc_jQuery("#tdSell").attr("innerHTML","<p>PURCHASE ADDITIONAL EVENT PASSES:<br><a href='https://www.e-junkie.com/ecom/gb.php?c=cart\&i=xxxxxx\&cl=xxxxxx\&ejc=2' target='ejejc' class='ec_ejc_thkbx' onClick='javascript:return EJEJC_lc(this);'><img src='http://www.e-junkie.com/ej/ejadd_to_cart.gif' border='0' alt='Add to Cart'/></a></p>");
}
