Steps in installing Oracle Application Express:

1. Download apex_3.0.zip file
2. Unzip apex_3.0.zip
3. Copy extracted directory “apex” to $ORACLE_HOME\
   ex. $ORACLE_HOMEC:\\oracle\\product\\10.2.0\\db_1\\
4.  Change working directory to apex
   ex. cd C:\\oracle\\product\\10.2.0\\db_1\\apex
5. start sqlplus and connect as SYS
   ex. sqlplus sys/ as sysdba
6. run apexins.sql with the following five arguments
   - password - password of the Application Express Administrator (ex. oracle10gdb)
   - tablespace_apex - tablespace to be used by Application Express (ex. SYSAUX)
   - tablespace_files - file tablespace to be used by Application Express (ex. SYSAUX)
   - tablespace_temp - temporary table space to be used by Application Express (ex. TEMP)
   - images - the images folder for Application Express (ex./i/)
   ex. @apexins password SYSAUX SYSAUX TEMP /i/

Oracle Application Express installs three new database accounts:

• FLOWS_030000
• FLOWS_FILES
• APEX_PUBLIC_USER

After a successful installation of Oracle Application Express you can see the following message:

Thank you for installing Oracle Application Express.Oracle Application Express is installed in the FLOWS_030000 schema.The structure of the link to the Application Express administration services is
as follows:
http://host:port/pls/apex/apex_admin (Oracle HTTP Server with mod_plsql)
http://host:port/apex/apex_admin (Oracle XML DB HTTP listener with the embed
ded PL/SQL gateway)The structure of the link to the Application Express development interface is as
follows:
http://host:port/pls/apex (Oracle HTTP Server with mod_plsql)
http://host:port/apex (Oracle XML DB HTTP listener with the embedded PL/SQL
gateway)

— Updating user account expiration. ———-
timing for: Upgrade
Elapsed: 00:00:00.15
JOB_QUEUE_PROCESSES: 10

Performing Application Express component validation - please wait…

Completing registration process.
Validating installation.
timing for: Validate Installation
Elapsed: 00:01:18.29
timing for: Complete Installation
Elapsed: 00:14:56.29
Disconnected from Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Pr
oduction
With the Partitioning, OLAP and Data Mining options

Source: Oracle Application Express Installation Guide

Note: If you have missing icon issue kindly try this one:
http://www.techxplore.net/2007/12/16/oracle-application-express-icons-missing/

Good luck.