According to their website, the location is at:
#17, St 114 Sankat Phsar Thmei 1 Khan Daun Penh, Behind Phsar Thmei
Tel: 012 777 990
081 828 267
But if you google map Street 114, you will find nothing.
That is probably because the street is known as Kramuon Sar.
If you google mamaks' corner, you will find nothing too.
Since Im a nice guy, I map the location myself:
View Phnom Penh in a larger map
So, have fun with Nasi Lemak! They've got this sambal petai sometimes, imported from Malaysia.