syncmods has been replaced by updates over SVN.
You have two choices at this point:
1) Wait for the next version udpate which will be downloadable through the blitz website
2) install a 3rd party SVN client (there's a bunch of free ones) and you can download updated through it.
Here's some instructions on what to do: (Probably a bit different for Linux, but at least it will give you some pointers:
http://www.blitzbasic.com/Community/posts.php?topic=76709SVN appears to be updated more often than syncmods was, but also be aware that it's a bit more 'experimental' than the official releases are.
If you do set up SVN, you may want to add Brucey's SVN repository as well, since it has a metric ton of other very useful Blitzmax modules on it.