Well let's try...
Since a server normally has no true type fonts you have to supply here. Some themes do offer fonts alongside, most will not.
Easy method : IN your theme (in use) create a folder named fonts..... upload a TTF font (eg copy one from within windows etc..) you like to use.
Now go to you rmedia manager and apply some settings where (at which size) watermark should be set :look at the following example.
Just some settings where for font i used an uploaded snowy font.
SAVE, and EMPTY cache.
Here's an example of how it looks when done..
HOWEVER !!! NOTE When watermark is set !! ALL images on site (larger than set size) WILL have watermark. So be carefull if you use it..