This is from Designer 2016 but guess it is the same in 2015.
Click on the Snap"mouse", hold just a little time so that the menu open and make sure the Middlepoint is checked, then move the cursor to about middle on the line, leftclick and it should find the middlepoint, i normally use the SEKE, then i select the dimensiontool, move the cursor to the line and press M(middlepoint), and then the next point in the end of the line, press V(vertex).
Also make sure that the SNAP down on Inspector Bar is not greyed out as this turn of all snap's too.
V18, V19, V20, V21, 2015/ 16/ 17 Pro. Platinum
Deluxe V20, V21, 2015/ 16/ 17
Windows 10 Home Premium 64 bit, 32 GB
NVIDIA Geforce GTX 780m, 1 GB