It's really relatively simple (it's probably the "relatively" that scares people!). Let me see if I can try to translate. Firstly you have to open a new tab (window) on the site you want to link to and copy the address from the address bar. Copy this by pressing the keys Ctrl and C at the same time.

Then, back on your post on XMarks, type in some text which you will use as and indicater of your link (example 'Click Here for Haggis') then select (underline) this (which covers the text in blue) by pressing MAJ and dragging the mouse over the text.

Once this is done, click on the icon (in the XMarks reply box which you are using) which is in the shape of a globe with a small sideways 8 on the bottom. This will open a window. All that is left to do is to paste the previously copied address (where you pressed the Ctrl and C keys at the same time) by pressing the keys Ctrl and V at the same time.

Easy, isn't it.