When I try to connect I get this error:
PKIX path building failed: unable to find valid certification path to requested target
I downloaded the certificate from this page and added it to KeyChain Access as you can see
I tried disabling ssl on the connection by setting ssl property to false, but that did not work.
So how can I connect using dbVisualizer?

I think you need to specify the location of the certificate file in the connection properties.
so at first locate the downloaded certificate file on your computer, then in
dbVisualizer, open the connection properties for theRedshiftconnection you are trying to establish.Now Under the
Advancedtab, replace "" with the actual file path to the certificate file you downloadedand done.