Maybe I am missing something... I can't seem to work out how to make Ambience do anything. I go into VST sound effects, change it to garritan, and put it on Concert Room 1, and nothing changes. There is no reverb or anything...
I just tried it to answer the question.
First you have to move the Garritan-Ambience .dll to the Vstplugin folder of Cubasis, as you did with the Garritan Orchestra. Cubasis loads (at installation) the VST plugins under the sub-directory Pinnacle. Look for Cubasis and VSTplugins. It should be there (together with Brother-Gregory and other plugins).
Load/import the Midi file. Press the button VST instruments and only load Garritan. Assign the GP to the various Midi channels. Next lower the volume for each of those tracks/channels (sometimes it is overdone and gives you a distortion from here to Tokio).
Press the button VST channel mixer and slide through the channels until the first occurence of Garritan, on my system the first V1.
Under the now window button edit you can insert the Garritan ambience. Be sure you have the button On lit (yellow color). You can also edit it the setting.
I am sure you have done this, but maybe for the wrong mixer channel.