Hi, I'm looking to build WiFi system for a hotel. One reseller in Bulgaria offered me this AP before 4 months with the corresponding controller and switches Cisco SG300-28P. We thought that they will output power to the all AP, but now after 4 months they're telling me that I will need only PoE injectors for powering them and I can't use PoE switches for powering them up. And now we're kinda very confused with choosing equipment. These AP must be installed in every room. And we have long time ago installed cabling system to every room.

Your reseller is correct in that you can't power mAP 2n directly from an SG300. The blame for the confusion lies squarely with Mikrotik who initially advertised them as 802.3af compatible but changed their minds once the first units were out in the field [see here: http://forum.mikrotik.com/viewtopic.php?f=3&t=88451]

So long as the cabling you've put in is Cat5 or better then you'll have no problem powering mAP2n.

If you want to power the mAP2n [or any other passive PoE device] from an 802.3af switch, you can use active > passive converters, which sit inline between the 802.3af switch and the passive PoE device.

Alternatively you could ask your reseller to exchange the 802.3af switch for one that supplies passive PoE...after all, it was they who told you it would work with what they sold you.