hi,
It appears Patch Release ESXi550-201404001 is to be used if your are on ESXi 5.5.
I am currently running 5.1 so i have to update to 5.5 using VMware-VMvisor-Installer-5.5.0.update01-1623387.x86_64.iso
That was found in this blog.
http://blogs.vmware.com/kb/2014/04/patching-esxi-5-5-heartbleed-without-installing-update-1.html
The info about the roll-up is in the README - ESX5.5U1-SuperISO-2-README.pdf
Important: The ESXi 5.5.1 Driver Rollup 2 is designed for fresh installations of ESXi hosts.
VMware does not support using the ESXi 5.5.1 Driver Rollup 2 to upgrade an existing ESXi installation.
The pdf can be found at the below link
http://www.tinkertry.com/vmware-esxi-5-5-update-1-driver-rollup-2-released-apr-24-2014/
So, I am not sure how the driver Rollup2 updates are applied.