The 5.1 Hardware Compatibility List (HCL) isn't showing the DS5020 as a supported array for AIX - which suggests this isn't supported, unless there is a technote that supercedes the HCL
Hardware Compatibility List for Veritas Storage Foundation (tm) and High Availability Solutions 5.1 (including SFCFS for Oracle RAC for RHEL, SLES and OEL)
http://support.veritas.com/docs/330441
If there is a technote/document indicating that it's supported using RDAC instead of MPIO, then follow IBM's instructions to disable MPIO and install/configure RDAC (along with any other required steps specified in the technote).
Edit: it was pointed out that the HW technote does list the DS5xxx as being supported using RDAC
Veritas Storage Foundation (tm) and High Availability Solutions Hardware TechNote for 5.0 MP1 and 5.0 MP3 (AIX, Solaris), 5.0 MP1 and 5.0 MP2 (HPUX) and 5.0MP2, 5.0MP3 (Linux), 5.1 (AIX, Linux, Solaris)
http://support.veritas.com/docs/283282
Per Dev's post below, as MPIO and RDAC are IBM products, contact IBM/refer to IBM's documentation to configure these components.