[Updated 4pm 30/06/2012]
OK, problem solved. There was a flag that enables the untitled activities to be shown. I have added in a few more search terms including date, distance (with an accuracy) and location with accuracy. The locations are pulled from a long/lat table on a separate sheet. I have begun to populate it with the locations of the parkruns. The latest version 5 is available here.
I would also like to that David Jacobson for his help with getting this up and running. David has a blog here which lists some more sophisticated software for looking at Garmin datasets in other ways.
[Updated 1:23am 30/06/2012]
I can't correct the failure to display other people non-named activities - it is a problem with the whole Garmin Connect system....but, I have a better searching facility on this sheet. It now implements distance, location and date searches. I have entered a few parkrun locations to test it.
[Updated 12:41pm 28/06/2012]
My previous version ignored activities without a name. I hope I have corrected that problem....
The Garmin Connect website is useful, but often rather slow and limited in terms of data manipulation. To overcome some of these problems I have written a short Excel VBA script that pulls down all of the activity IDs and GPS files for any user (not just your own). The Excel file is here. Save the file somewhere sensible (don't open it from the link) and when you open it, enable macros. You need to enter your own Garmin Connect detail (username and password) and the user you want to download for. Then click the extract button.
The code is all visible, but not documented - anyone with a small amount of VBA experience should be able to tweak it for their own needs.