Firstly, you need to install Synth1, make sure you point to the folder you have chosen as the folder for your VST-instruments! Atm we recommend you to make a subfolder in the folder Samples -like
C:\Users\<name>\Documents\lmms\samples\VSTinstruments
The reason is that they will be accessible from subfolders, letting you get a nice clean installation:
C:\Users\<name>\Documents\lmms\samples\VSTinstruments\Synth1
So thats the installation target
Then you just follow all steps in this video:
Let us know the outcome :)