perhaps a step wise instruction
1) Close PellesC
2) Copy the dll into the "Bin" folder subfolder "Addins64"
3) Open PellesC
4) Click 'Tools'
5) Click 'Customize'
6) Click 'Add-Ins' tab
7) Check box next to "PellesC Line Counte Add-In" (I see typo)

Click OK
Steps 1-8 only need to be done the first time
9) Open a PellesC project file
10) Select a source .C file
11) Click Menu 'Source'
12) Click 'Line Counts' - Should be there

like attached screen shot
13) See results in Output tab
John Z