Reference tags inside Matrix textfield
You'll need to use the global set's element ID as the reference part (second segment) of the reference tag:

{globalset:51:siteUrl}


To find the global set's ID, go to CP -> Settings -> Globals. The URL to the global set will have the ID as the last segment (e.g. http://yoursite.craft.dev/admin/settings/globals/51)

Also, see the this SE thread for a little more info: How can I use a reference tag with a global set using the handle (not the ID)