(I am summarizing the comments in this answer so that it can be accepted. Thx.)
The "default instance" is generally the one with no name. "default instance" doesn't mean "only one on the box". To make it more worse, sometimes MS refers to the unnamed instance as "MSSQLSERVER". I'm guessing that the SSRS instance that you are naming "MSSQLSERVER" is looking for a default instance of SQL (to store the SSRS metadata) I would try installing SSRS as SQLEXPRESS and see what happens.
To be fair, I've never run SSRS on SQLEXPRESS, but some pages over at MSS seem to indicate that it works, with some caveats (like no scheduled reports because there is no SQL Agent).
After installing SSRS, you may still need to manually configure SSRS.