JW,
Sorry for not getting back to this. Other bugs or not, there's no excuse, and I apologize for that.
As for your issue, you will need to redirect the user *somewhere*. So, here's what I would recommend doing:
* Install a custom module that lets you insert PHP. Something like
Flexi Custom Code
is a good example, but there are others. I've used that on one site in the past. I used to recommend something different for J1.5 though, so I don't really know if there are better for J2.5. Anyways..
* Create a new module that you can insert PHP into it. Add the code you mentioned in your post above. With that, you will have a module that should create a modal popup on the page when that page loads that module.
* Assign the module to position "custom_welcome" (you'll have to type that into the position box).
* In the plugin manager, make sure the Content - Load Modules plugin is enabled.
* Create a Joomla article. Title it "Welcome page" and you may put some text in there if you want... or not.
* In the article contents, include the following tag: {loadposition custom_welcome}
* Create a new menu item that points to the article you just created. You may need to make a new menu altogether and simply not load it into any module positions. That will allow you to create a menu item for the article, but not have it displayed anywhere for the user.
* In JLinked/JFBConnect, set the new user redirection to go to this new menu.
Whew! Obviously, you can test the article directly by going to it before the redirection is setup. Then, once that works, test it out with the redirection to make sure it all works together. Lots of steps, but honestly shouldn't be too bad to implement.
Let me know if you have any questions or concerns, and sorry once again!
Thanks,
Alex