This is how I got AAC+ streams playing on my SqueezeBox, the link is step one is a direct export of the files of the directory on my server, so it should be guaranteed to work. If not, I got my information from here.

1. Download this
2. Unzip the contents so that in the SqueezeCenter7 directory \Program Files\SqueezeCenter\server\Plugins
3. Restart Slimserver and check Settings/Advanced/FileTypes that there are AAC or AACplus entries and they are enabled.
4. Test