You can find the scripts to create the table in the Enterprise Manager installation.
Under EM_HOME/install/oracle/database-scripts, you can find the script createtables-oracle-xx.x.x.x.sql, for example createtables-oracle-10.5.0.0.sql
CREATE TABLE apm_secure_store(
alias VARCHAR(256) NOT NULL,
cipher_text CLOB NOT NULL,
created_date TIMESTAMP NOT NULL,
last_read_date TIMESTAMP,
client_id VARCHAR(256) NOT NULL,
user_id VARCHAR(256),
PRIMARY KEY (alias, user_id)
);