I upgraded to El Capitan on Friday. I installed XS today:
Version 5.10 (build 913)
Installation UUID: 36314227-811f-4808-b811-644922ae7081
Mono 4.2.1 (explicit/804ddbc)
GTK+ 2.24.23 (Raleigh theme)
Package version: 402010060
On the command-line, I can access mono and fsharpi:
$ which mono fsharpi
When I attempt to view the F# Interactive pad, I receive:
Session termination detected. Press Enter to restart.
/Library/Frameworks/Mono.framework/Versions/4.2.1/bin/fsharpi: line 24: exec: mono: not found
If I press "Enter" to restart, I receive the same message.
*** This bug has been marked as a duplicate of bug 34470 ***