Thanks for the suggestions, Saar, I have got it work now
Here are my detailed instructions for getting Snagit to connect to Gemini.
Unzip the SnagitAddon files into a subdir of your Program Files directory, eg
C:\Program Files\CounterSoft\CounterSoftGeminiSnagitv37_0
Open the Snagit Editor, Send tab, click on the little down arrow in the bottom right corner of the Output Options panel.
In the Output Properties dialog, select the Program tab.
In the list of programs Edit the item named 'CounterSoft Gemini TechSmith Snagit Integration'.
Set the executable to point to the CounterSoft.Gemini.SnagIt.exe found in the Program Files location where you unzipped the files.
By default the Parameters field has "<CaptureFilename>" as the only entry.
Add after this the name of your Gemini site, your username and the API key.
It should look something like this "<CaptureFilename>" http://mycompanyname/gemini fbloggs xxxyyyzzz
Note that there are no brackets or quotes around the last three items whilst the first item should stay as the default has it.
Click Ok and Ok to save your changes.
To test this, save an image in Snagit so that there is a filename displayed in the title bar of the editor.
On the Send tab, select Program from the Output Options, and then select 'CounterSoft Gemini TechSmith Snagit Integration'
from the list of programs.
The Capture dialog should now display.
The security credentials should show with the details that you put in earlier.
The Snagit File field is greyed out but should show the path and filename of your selected file.
If you don't see the filename in the File field then you have not setup the parameters properly.
Fill in an Issue Id and comment and click Capture.
Note that the IssueId is just the unique number of the Gemini Issue, it does not include the project prefix.