Got my Spark Core today. I tried many times but I could not get the Android app to connect even though the Spark Core went through the correct sequence of light blinks and colors and was throbbing cyan. Searched these forums for "asus" (my wireless router is an Asus RT-N56U) and found this thread that gave me some ideas (thanks, neuromancer):
On the Wireless General page I changed the Wireless Mode from Auto to Legacy and now the Android app connects. The on-line help tells me that Legacy mode supports 802.11b/g/n just like Auto mode but limits 802.11n to 54Mbps. This is the first time I've had a WiFi device not connect to this router. Hope this helps others.