Merhaba Hakan,
Welcome to the community! Feel free to Introduce yourself 
to the community if you want to! 
Are you trying to do this in Excel? Our Power Query connector does not work in Excel as Excel Power Query can not have custom connectors. It is Power BI only.
Are you trying to extract the value Pset_MTH? If so, this should be pretty straightforward.
As Pset_MTH key is stored in the top level, accessing it is pretty easy. When you go through Add Column > Custom Column, use [data][Pset_MTH] formula.
Elements that have that field will return the Record, others will throw an error. You can remove the error rows and proceed to extracting all fields of the records.
I’ve attached the Power BI file. Please take a look and let me know if you have any questions.
15687 - Pset_MTH from IFC.pbix (35.6 KB)