Ambience reverb is not available from the Banks & Effects panel in recent versions of Finale (only from the Effects tab of the Aria Player) but I do not understand why you cannot apply in in Finale 2009. I am mystified.

Is it possible that when you installed the Garritan System on your new computer that it only installed the version of Aria (and Aria Ambience) used with 64-bit versions? Presumably the latest versions of Finale have a way to access what are likely still 32-bit sound files, but I'm not sure about the Ambience & Convolution Reverb available in the latest Garritan Players (which come in 32-bit and 64-bit stand-alone versions).

I've had a similar problem and found that increasing the Send value in the Aria player for instruments usually puts Ambience and Convolution into action.  This is probably a glitch in the aria player.  I have the standard Garritan library plus the Classic Pipe Organs and Personal Orchestra library.  It seems to occur most when using the Personal Orchestra library.  The other libraries seem to have a default value for Send that is sufficient for all (at least most) instrument choices to play within the Effects range (i.e., Send value greater than 0).  It's really sad that we pay so much money for these libraries have to fiddle with them to get them to perform as intended.  Garritan needs a little better quality control.  There also needs to be identical settings in Finale for the Aria mixer settings since Finale, by default, does not allow alterations to many of the Aria settings to override Finale definitions (particularly with Human Playback enabled).  After using 3 or 4 versions of Finale, I've given up expecting the software engineers to eliminate these annoying little issues.  One thing I learned when first learning computer programming in the 1980s was "GIGO" - that is, garbage in, garbage out.  This fundamental principle needs to be in the forefront of every developer's mind at all times....just saying...

Sorry, I reject the idea (expressed by many users of many different software packages) the "because this program is so expensive Company X should have programmed my personal preferences into it as the default."

Finale provides considerable flexibility to users to configure the program as they wish (to set different send levels to different instruments within Aria reverb to reflect their distance from the audience). That means adjusting settings to reflect how each user wants his or her scores to appear or sound. One of the reasons I seldom use DAW's is because I am not willing to do the work they require. That is the nature of such programs; I don't expect the DAW company to turn its front-line programs into a kiddie version because I am not wiling to invest the effort into learning how to use them.

Rant over. :-)