In case you bought a mikrotik device with embedded license, it is not possible to rise license level. If you have “lite” device with L3 and you need to use it as AP, there is no way with Mikrotik ROS.
Many years ago it was possible on some very old version of ROS to create a VirtualAP on L3 device. But it is not possible now and I doubt that such old ROS would run on SXT Lite.
There are maybe some ways how to install DDWRT or OpenWRT or similar open linux distributions instead ROS, then you would be able to use such device as AP. But I have not done it and I have no information if it works and how it works - try to google and look at supported devices lists of these systems, as on this forum there is no support for it.
I would suggest to buy correct device with L4 licence that covers your needs and sell or use the client level device for client/bridge connection. It will be finally cheaper then finding and trying some obscure ways how to go around.
Maybe try to ask your vendor if he is willing to swap the devices for you - with compensation - as it is new an unused…
Just and remark for others. Read carefully the descriptions and distinguish at least between L3 and higher levels of license. L3 is not usable as AP! Only the higher levels are. If your vendor told you you can use SXT Lite as AP, then throw it on him back with full refund. If it is your fault, try to negotiate… 
Edit:
Mikrotik devices supported by OpenWRT: http://wiki.openwrt.org/toh/start#mikrotik
OpenWrt support patch for SXT Lite: http://patchwork.openwrt.org/patch/5491/
Other patch for SXT Lite: http://patchwork.openwrt.org/patch/5547/
maybe more patches exist… http://patchwork.openwrt.org/patch/5746/
Remember that you can definitelly damage your device by trying to install 3rd party software. If you try, please, share your experience on this forum.
Edit2:
There is thread for OpenWrt here: http://forum.mikrotik.com/t/openwrt-for-routerboard/50383/1