Install: zenCart 1.5.1e

PHP 5.4.0

No plugins to speak of

I was getting the standard problem of cards being processed but orders not completing in the cart from using the old SIM module. Today I installed the new module and configured it (at least, I think it's configured properly). However, when I run test transactions, they hang up on "Step 3 of 3", not completing load of main page and not loading right column of layout at all. The last thing that loads is the order total. The window does not pop up to allow credit card information to be taken on's website. (I tried flipping over to "collect card info here", but that didn't process either.)

My host (HostGator) only offers 1.5.1 for fresh installs and doesn't do upgrades, so if the new module is incompatible with anything below 1.5.5 I'm kind of stuck...