If my understanding of your post is correct the correct procedure wasn't followed.
In order to install Ubuntu MATE you need to have a BOOTABLE usb stick made CONTAINING the iso.
Once the usb stick has been made bootable with the iso you've already downloaded you boot the usb stick which should provide options to try or install.
Steps to follow:
1 - download iso and save to any place other than USB stick you from which you intend to boot / try / install [no need to download again ... simply copy the iso file to your hard drive]
2 - burn the iso file to the usb stick you intend to use for test / install
3 - boot the netbook using the usb stick bade in step 2
4 - follow @wolfman's guide from part way down look for text "Okay, I've changed the boot order but I don't know what to do next?."
5 - test your netbook; if satisfied and you've decided not to use Win 7 on this netbook for installation type choose Erase disk and Install Ubuntu MATE
6 - you may ignore pics 4-10 in the guide
7 - pic 16 on will apply
Please post back with either more questions, or success 