While unsure which brackets you got, you will need both – the right-angled ones and the ones holding the shade's headrail.
Or, you can use an angle iron (aluminum) to span across the window width just right below the arch.
Usually, most hardware stores, even Home Depot will have some sort of angle iron available.

One way or another, though, you'll still need both types of brackets to install the "holding" part.

Take a look at the pictures on these pages for visual help:

http://myhomewindowblinds.com/06/17/how-to-find-a-correct-bracket-position-for-an-installation-of-window-blinds-and-shades/
http://myhomewindowblinds.com/07/18/how-to-install-blinds-and-shades-in-a-wide-span-window/
http://myhomewindowblinds.com/05/05/window-blinds-and-shades-side-mount-example/

Here is another example how to install them:

The picture above shows a Hunter Douglas shade – side-mount.

There are two brackets: the ones that come with the shade
(you can see the gray bracket with a small screw)
and the metal, right-angled bracket attached to the window.

Looks like you may need the ones that go with your blind
or shade – and attach to the metal bracket, correct?

1.
Take a look at the sticker to find out who makes your blind or shade.

2.
Possibly take a snapshot shot of your shade and find the nearest
dealer for that manufacturer. If need be, send them the photo
of your blind or shade (to make sure they know which "model" it is).

3.
Once the dealer has the info, they can help you get the brackets you need.

Pete

Try Home Depot – The Blinds Dpt. – Either Levolor or Hunter Douglas
right-angled brackets will do (if they have some).

If you have to, just grab any suitable right-angled brackets in the store
and drill holes into them so that you can attach the shade brackets
with a couple of screws and nuts.

And…then use a couple of wood screws to attach the bracket assemblies
to the window frame.

Make sure you mount them level on both sides – measure (the height of the shade and use
a level to make it easier.

Pete

Thanks