Oracle 10g dump file into Oracle 11g database
That error is most likely due to incorrect environment settings indeed, probably your PATH is mixed up too.

In a command prompt, assuming that your Oracle 11g home is indeed as you posted above, run:

set ORACLE_HOME=C:\app\Administrator\product\11.1.0\db_1
set ORACLE_SID=ORCL11R1
set PATH=%ORACLE_HOME%\bin;%PATH%


Make sure you can connect with sqlplus, then start imp.

(Do consider using the expdp/impdp Data pump tools instead though. imp/exp are essentially deprecated in 11g.)