Changing the default schema of a user
Sounds like you don't have SELECT access on the sequence. Ask the DBA to run:

GRANT SELECT ON SCHEMA_OWNER.MY_SEQ TO APP_USER;

Can you edit your post with the full error message you're receiving? 

Regarding your comment. I suspect you've got SELECT ANY TABLE etc privs. These don't apply to sequences, as demonstrated below:

PHIL@PHILL11G2 &gt; create user test1 identified by test1;

User created.

PHIL@PHILL11G2 &gt; grant connect,select any table to test1;

Grant succeeded.

PHIL@PHILL11G2 &gt; create sequence testseq;

Sequence created.

PHIL@PHILL11G2 &gt; conn test1/test1;
Connected.
Loading glogin.sql

Session altered.

TEST1@PHILL11G2 &gt; select count(*) from phil.bigtable;

  COUNT(*)
----------
   2300640

TEST1@PHILL11G2 &gt; 

TEST1@PHILL11G2 &gt; select phil.testseq.nextval from dual;
select phil.testseq.nextval from dual
            *
ERROR at line 1:
ORA-01031: insufficient privileges