If you get the "Message file sp1<lang>.msb not found" message when trying to run sqlplus and instantclient on an AIX machine, check that you have a LIBPATH environment variable pointing at the "instantclient_10_2" directory. If you're like me, you want to make that environment variable LD_LIBRARY_PATH (as it is on Linux), but on AIX it's just LIBPATH.
These were my environment variables for getting sqlplus connecting to a remote Oracle database from an AIX box with just Oracle Instant Client installed:
This is useful information!
Let me add my remarks:
1) ORACLE_HOME is not necessary for Instant Client.
2) What is the background of this error message?
The Oracle client shared library (libclntsh.so.10.1 or oci.dll or similar) is the same for Instant Client and the regular client.
When accessed, it tries to load the Instant Client library (libociei.so or oraociei11.dll or similar).
If that succeeds, it considers itself an Instant Client; if that fails, it thinks it is a normal client.
The main difference between Instant Client and the normal client ist that the former takes all its message files from the Instant Client library, while the regular client has its message files in the file system.
This is what probably happens here: The Instant Client library was not found, the client thinks that it is a regular client and tries to access the message files on the file system.