I would try a couple things.
1) Hit F9 and open your 'System Browser'. Make sure you do not already have a sanitary system under the plumbing. I have ran into this issue where I cannot lay out a path b/c my earlier attempts were done incorrectly.
2) Click a plumbing fixture and instead of 'Add Equipment', click 'Edit System'. Pick Sanitary, then 'select equipment' from the left design bar.
3) Change your view range to view everything. Click a plumbing fixture, then the sunglasses at the bottom view control bar. Select 'Isolate category'. Click the sunglasses again and hit apply/....etc.
Let me know if any of this helps.