how do i download iso for 10.10 and install in macbookpro late 2008

Click on this link.

How to upgrade to OS X Yosemite – Apple Support

Go to Section 4 and click on Download OS X Yosemite.

This downloads InstallMacOSX.dmg to your Downloads folder.

The next section can only be done on a mac that is capable of running Yosemite,

a mac that came preinstalled with an OS later than Yosemite will refuse to do the next bit.

When downloaded open to InstallMacOSX.pkg, double-click on

that and an installation window will open, this does not install Yosemite

but converts the InstallMacOSX.pkg to the Install OS X Yosemite.app which 

you will find in your Applications folder.

 (If the installation window asks which disk you want to install to, you must pick 

the disk that you are booted to at the time. Not any internal or external disk that 

you want to eventually install Yosemite on, that is for later.)

To start the installation of Yosemite double click on the Install OS X Yosemite.app.

The copy of the install app self deletes after installing Yosemite, so make sure you keep a copy of the InstallMacOSX.dmg if you need it in future, or you could just make a copy of the Install OS X Yosemite.app

prior to installing and moving it to an external drive for safe keeping. It is also possible to create a bootable USB installer disk using the Install OS X Yosemite.app in the Applications folder and the createinstallmedia command in the Terminal app. 

Read the instructions here,

How to create a bootable installer for macOS – Apple Support

sudo /Applications/Install\ OS\ X\ Yosemite.app/Contents/Resources/createinstallmedia --volume /Volumes/MyVolume --applicationpath /Applications/Install\ OS\ X\ Yosemite.app

You need to create this using the instructions I gave you.

And use Disk Creator, select the USB stick you want to use.

And then select the Install OS X Yosemite.app.

When ready press Create Installer.

Have you managed to download InstalMacOSX.dmg and then create the Install OS X Yosemite.app

in the Applications folder.

Then you are ready to make the bootable USB using the Install OS X Yosemite.app,

read the instructions in the link above.

Or use this free app, https://macdaddy.io/install-disk-creator/

What is the point in me writing out a fully explained post the procedures you need to go through

to get Mac OS X Yosemite onto a bootable USB installer if you are not going to read them

and act on them correctly.

My first post clearly says to download Install MacOSX.dmg.

Open that up to get InstalMacOSX.pkg.

Open that and you will get an installation window where

InstallMacOSX.pkg is converted to Install OS Yosemite.app

which you will find in the Applications folder.

Use Install OS X Yosemite.app and the Disk Creator app

to create your bootable USB stick.

Go back to my first post and read, read and read again.

Don't tell me it doesn't work, it does, I have done it many times.

Follow my instructions and not your own interpretation.

It is simple, do it.