How to downloed 10.6.7 dmg file

There is no such file available. That version was only available on the Software Restore DVDs that came with your computer when it was new. Apple no longer sells or supplies any versions of Snow Leopard. You will have to look for it elsewhere. If your model requires 10.6.7 in order to start up, then you will need to find that exact DVD elsewhere. However, you can still buy and download Lion. You can purchase Lion at the Online Apple Store. The cost is $19.99 (as it was before) plus tax.  It's a download. You will get an email containing a redemption code that you then use at the Mac App Store to download Lion. Save a copy of that installer to your Downloads folder because the installer deletes itself at the end of the installation.