The Alleycat's Neihu map, although basically right, is slightly misleading. As you head east on Minquan road, cross the Minquan bridge, go past Costco's, past Ruiguang St., past Chenggong Rd. (that's right, Minquan and Chenggong meet in TWO places ), through the Minquan tunnel, and after a small light or two, you'll hit a major intersection, which is the one displayed on the Alleycat Neihu map, but if I'm not mistaken:
1) the orientation is about 70 degrees off; at that point Minquan has curved and is not running due E-W (minor point), and
2) a left turn puts you not on Kangning Rd but on Jinhu Rd which then continues up to near the De2An1 dept. store in Neihu; it's a right turn which instead puts you on Kangning Rd.. The problem is that at this intersection the road names change in every direction (Minquan turns into Chenggong -- yes, Chenggong), and Kangning turns into Jinhu. This is particularly relevant if you're going there from the DeAn or Dahu Park areas.
Anyway, at that intersection as you face east, with the tunnels to your back, you'll see MOSburger to your forward left (NE), behind which is Alleycat's, hidden in a small "alley" (actually a pedestrian sidewalk between two buildings).
If you're going there from Nangang, just head north from the Academia Sinica, following the MRT construction along Kangning Rd; go past Donghu Rd. (where the Hala theater castle, McD's & KFC are), past a few minor lights, to the Minquan intersection. Cross it, and stop on the right.
Hope that helps, and enjoy the great 'za!