Ahh, sounds like you put wayy to much on. We've all over-tarred our rims at some point. Firstly, to fix it, sand the top layer of tar off your pads and clean your rim with white spirit (Unless your running an anodised rim. I have a feeling the white spirit will make it come off but not sure), if you have anodised, try and clean it with hot soapy water then make sure you get ALL of it off after by rinsing it with water. However, if you don't care about your sidewalls or it's already been ground etc. White spirit works amazing. Just put some on a paper towel or something and scrub the rim. The tar basically flies off. Secondly, putting the tar on. You don't want to end up doing the same thing again. Theres two main methods for putting it on. Most people use this one. Spin the wheel and lightly touch the tar on the side of the rim for a second or two, then do the same to the other side. I don't like that way though because I can never get the right amount on. All do is, with the wheel still, just put wee lines of it at certain parts of the rim. So about 3-4 lines on 1 side then the same on the other. After you've put it on cycle around with your brake on for a few seconds to spread it round the rim. It will make an awful racket but you only to do it for a few seconds. Just experiment with it but making sure you don't put too much on. Hope this helps.