What model of Mac do you have? Does it meet the minimum system requirements for Snow Leopard?
If it does, is this disk a retail copy of the 10.6 installer (white label with a picture of a snow leopard) or is it the install disk from another model of Mac (grey lable). If the latter, that won't work.
If you can give us more information, we'll be in better position to offer further suggestions.
These are information of my Desktop:
Model Name: iMac
Model Identifier: iMac4,1
Processor Name: Intel Core Duo
Processor Speed: 2 GHz
Number Of Processors: 1
Total Number Of Cores: 2
L2 Cache: 2 MB
Memory: 512 MB
Bus Speed: 667 MHz
Boot ROM Version: IM41.0055.B08
SMC Version (system): 1.1f5
The disk a retail copy of the 10.6 installer ,white label with a picture of a snow leopard new version 10.6.3.
That model can accept 2 -1GB RAM modules (recommneded), likely the access port is underneath the display.
The maximum OS X verison it can use is 10.6.8 installed via the 10.6.3 disk you have. No 10.7 or later versions.
I advise backing your personal data folders (Music, Docs, Pic's etc) to a external drive (via drag and drop copy methods) first before doing anything, disconnect this drive while updating OS X to prevent errors.
Once the RAM is installed, (Use the Crucial.com online checker for the correct RAM specs, videos on You Tube how to install) reboot twice and the second time stick the 10.6.3 disk in and run the installer/upgrader.
If it doesn't work you will have to follow this proceedure to erase and install the entire drive then install 10.6.3 fresh.
Use the same user name when setting up.
Then Software update under the Apple menu to 10.6.8
Install 10.6 compatible third party software, then files from the external drive into the same named account.
Erase the external drive and use it to set up a Carbon Copy Clone (bootable), or a TimeMachine drive as backup. (not bootable)
Once 10.6 is installed and updated, the SMC firmware is changed and you will no longer can use the 10.4 or earleir boot disks on that Mac again.
I insert 2 of 2GB DDR2 RAM memory, one in each module, but it doesn't work after reboot the computer. I took one 2 GB and let only one 2GB in the slot and the other slot empty and reboot and is working as before. its show 2GB memory, can i update 2 GB more in my computer for a total of 4GB RAM memory? thank you.