cant install new plug ins

Hi, I have been trying out a few plug ins on my site, couldnt find what i wanted, so had someone create one for me. It was just a simple portfolio.

Now, My site is live, & I want to install some more plug ins, but keep getting errors.. I tried to update some already on there & get this error as well. any ideas?
“download failed. couldnt resolve host “downloads.wordpress.org”
Is it a permissions thingy? what can i do?

many thanks