MACUPDATE ADMIN  Just now downloaded the file, the .dmg mounts with no problems. Issue appears to be at your end.  
(Version 1.0)

ALLBLUE  I'm running 10.3.9 - is it 10.4 and above? Would that give me the "no mountable file systems" warning when I open the dmg?   
(Version 1.0)

MACUPDATE ADMIN  No. You probably just need to restart your Mac. On the very rare occasion that a .dmg will not mount for me, a restart does the trick.  
(Version 1.0)

ALLBLUE  OK I tried restarting, but with the same end result - I double-click the dmg icon, 'agree' to the EULA and get the same "no mountable file systems" message. How strange. One odd thing I noticed (this happened both times I downloaded previously, but I couldn't take details of the file name) is that when I click on the download link above Firefox asks me what I want to do - meanwhile the file rbjayzwz.exe (complete with ugly three-barred icon) appears in my downloads folder. When I ok FF to download and save the dmg the .exe disappears and the dmg takes it's place. I don't understand that at all - what's an .exe file got to do with anything?   
(Version 1.0)

MACUPDATE ADMIN  Use another browser to download the file. There is nothing wrong with the downloaded .dmg.  
(Version 1.0)

ALLBLUE  Thanks for your advice btw, but it's not really helping I'm afraid! I just started Safari, downloaded, agreed to the EULA, disk image tries to mount... mounting failed appeared in the download window. I haven't got a dmg problem generally, I only just downloaded Skype which mounted as normal, but for some reason this one just will not.

Oh well it's not the end of the world, it does look like a useful addition to PS but if I can't get it I can't get it!  
(Version 1.0)

MACUPDATE ADMIN  All I can tell you is that I have downloaded the file three times today to check it. Each time it downloads successfully and the .dmg mounts with no issues.  
(Version 1.0)

ALLBLUE  Well it downloads ok, it even verifies ok, it just isn't able to mount. It clearly is a valid file if you are opening it, I normally don't have a problem mounting dmgs so I am completely at a loss as to what could be happening here.   
(Version 1.0)

MACUPDATE ADMIN  Well, you might want to contact the developer and ask to make the file available in .zip format...  
(Version 1.0)

CéDRIC RICCI  Hi,

You can download an archived (Zip) version at this address:

http://www.riccicedricdesign.com/ActionEffects/files/Riccicedricdesign Action Effects 1.0 ENG.zip

Sincerly,

Cédric Ricci

www.riccicedricdesign.com  
(Version 1.0)

ALLBLUE  Thank you Cedric, it has all worked as a zip, so I shall have a play with it over the next couple of weeks. The dmg thing is a bit of a mystery, I can see no reason why it would fail to mount. It is a repeatable anomalous behaviour and has left me baffled...  
(Version 1.0)