To use the Serena StarTool disassembler:  

1) While in a member list for a load library: use line option CS (CSECTS) 
2) Then use line command di (disassemble) next the CSECT you want to disassemble. 

Here is an example for SYS1.LINKLIB for member IEFBR14 and CSECT IEFBR14: 

EDIT       ------>DISASM   IEFBR14           MOD(IEFBR14 ) Colum
Command ===>                                                  Sc
****** ***************************** Top of Data ***************
000001
000002 ** DISASM   IEFBR14
000003 AT 000000  CSECT IEFBR14   LENGTH 000004
000004 000000 0000  IEFBR14  CSECT
000005 000000 0000           SR    R15,R15                 1BFF
000006 000002 0002           BR    R14                     07FE
000007
000008 7 blocks in this member
****** **************************** Bottom of Data *************