What Solaris version you use ? ( 11 ; 11.3 11.4 ? )
Show result of command: pkg info entire
What version STB you install and what is source ?
Standart path for explorer is : /opt/SUNWexplo/bin/explorer
Check also this docs:
How to Install/Upgrade Explorer in Solaris 11.3 and 11.4 (Doc ID 2122392.1)
Oracle Services Tools Bundle (STB) - RDA/Explorer, SNEEP, ACT (Doc ID 1153444.1)
As Nik suggests, knowing which version of Solaris 11 you are using and where you're installing explorer from is key. From 11.3 it's installed by default with most default group packages. /usr/sbin/explorer is a link to /usr/lib/explorer/bin/explorer so either should work