HOST is an SQL*Plus command, not a PL/SQL command. To run host commands from PL/SQL you need to use a Java stored procedure or a shared library via EXTPROC. Java is the easiest option:

http://www.oracle-base.com/Articles/...sFromPLSQL.asp

If you want to return host output consider using something like:

http://asktom.oracle.com/pls/ask/f?p...0_P8_DISPLAYID,F4950_P8_CRITERIA:3069633370832,%7Bshell%7D%20and%20%7Bcommands%7D%20and%20%7Bfrom%7D%20and%20%7BPL %2FSQL%7D

Cheers