You probably didn't use the Dell customized ESXi ISO when you installed your host. As a result, ESXi doesn't have the lsi-mr3 driver, which is the driver required to support that storage controller. I'd recommend you re-install your host with the proper Dell ISO that provides this and other drivers/utilities that are necessary.
If Dell is unable to provide the ISO, how do I manually install that driver?
To download the Dell EMC-customized ESXi ISO images:
1 Go to Dell.com/support.
2 Type your system Service Tag and click Submit.
3 On the Product Support page, click Drivers and Downloads.
4 On the Drivers and Downloads page, click the link Click here for other operating system.
5 From the Operating System drop-down menu, select the ESXi that you have to install.
6 You can view a list of categories, click Enterprise Solutions.
7 Download VMware ESXi 6.x