Based on the comments below your question, it sounds like you're trying to connect from your Veeam virtual machine directly to the MD3220i virtual disk with iSCSI (not managing the iSCSI connections from the ESX host).
If your Veeam host is running Windows and is using the built-in Microsoft MPIO (or doesn't have MPIO installed at all) that may explain why you would see the 19-20MB LUN(s).
Any hosts connecting directly via iSCSI - whether physical or virtual - will need an installation of Modular Disk Storage Manager run from the resource DVD. You can run this as a host-only install to skip the management software installation and only put MPIO in place, but the management software and MPIO drivers are all packaged on a single DVD.
If you no longer see the "access" LUNs (19-20MB) listed after installing the MPIO drivers (which is good) but still don't see your 5TB backup volume, you may need to revisit your host mapping configuration for the volume - it can be a little confusing the first few times you attempt it, but the listing for your host should show it has the Access LUN and your 5TB LUN both mapped to it (they'd be on the same list, one above the other when your host is selected).
Latest resource dvd:
http://downloads.dell.com/FOLDER03887874M/1/DELL_MDSS_Consolidated_RDVD_6_4_0_5.iso
User guide (see p56 for host mapping info):
ftp://downloads.dell.com/Manuals/common/md32xx_md36xx_administrator%20guide_en%20us.pdf