The archaeological sites are not open for public (at least at the time of writing). They are not anything impressive to see either as they are just grassy hills with some makeshift buildings for the staff that (I suppose) work here. If you insist on going to the Hongshan site, there is a bus from the central station in Chifeng. You will be disappointed ;)

Instead head to a museum to see these. I know it's not "the same" but the Chinese basically move all the artifacts there. Take it up with UNESCO/ICOMOS whether that is kosher. Nevertheless the museum in Chifeng has a nice, free, permanent exhibition of the Hongshan culture as well as several other significant local history such as the bronze age findings, Mongolian ruling time. Top this up with nice English displays and the ~1h spend in here is worth it. Note that the opening times include a long lunch break so time your visit accordingly.

To get the Chifeng you can easily take a bus or train from Beijing (6-7h), and if you are already up in the area you should check out Xanada (Shangdu) over at Duolun. The bus is around 4-5h and you can go to Beijing from here to do a nice triangle trip and "tick off" two historic sites.