This guide is only for connecting osu! to Akatsuki, and not setting the game itself up. You can follow this guide to set the client up.
Alternatively, you can edit your hosts file manually. To do so, run
nano /etc/hosts as root/with sudo.
When you’ve got it open, paste the following at the bottom:
126.96.36.199 c.ppy.sh ce.ppy.sh c3.ppy.sh c4.ppy.sh c5.ppy.sh c6.ppy.sh a.ppy.sh i.ppy.sh osu.ppy.sh 188.8.131.52 bm6.ppy.sh
CTRL+X and then Enter to save the file.
Download the certificate by clicking here
Open the Internet Explorer configuration by running
Double click the Internet Settings icon, navigate to the Content tab, then click the Certificates… button.
Click on Import, then Next.
Click Browse… then select the Akatsuki certificate.
Select Place all certificates in the following store, and click Browse.
Select Trusted Root Certification Authorities, and click Ok.
Click Next, Finish.
You should get a message saying The import was successful.
After that is done, you can start the client up, and log in with your Akatsuki credentials.