Q: can't access to recovery mode

Recovery mode is only available to Macs with the proper firmware.   All Macs older than the July 20, 2011 release of Lion, require a special firmware for recovery mode:

Computers that can be upgraded to use OS X Internet Recovery - Apple Support

Based on that article, your 2008 Mac is too old to even get that recovery mode.  You'll need the original installer discs for recovery.

Call AppleCare if they are missing.  A copy of 10.6 retail can also be used to reset the password, and that you can buy from the Apple Store:

http://store.apple.com/us/product/MC573/mac-os-x-106-snow-leopard

If trying to just reset the password of the primary user, you can use single user mode to reset the password.  That means booting with command-S, and following these directions:

Resetting the Admin password in single user mode

Note older Apple articles mentioned in this tip may not work, and you should use this tip to convert the older articles to new ones:

Knowledgebase links of the info.apple.com format no longer work

Is this what you are trying to do?

Boot to the Recovery Volume (command - R on a restart or hold down the option/alt key during a restart and select Recovery Volume).

User Password Reset