Until you get a good basking (heat) light over the basking area and get the basking temp up to around 90 and get the water temp down to around 78 (or 75 if this is a grown turtle 4" long or longer), your turtle won't bask. They are naturally attracted to heat and the difference between the water temp (cooler) and the basking temp (warmer) is what motivates them to get up and bask. If the basking area isn't warmer, no matter how nice it is they won't bother to get up and bask. And you can't put them up there and expect them to stay. They have to want to get up there on their own.
Have you read up on lighting? You'll actually need two light bulbs - one for heat and one for UVB. Here's a link that talks about it. If you have any questions come back here and we'll try to help.
http://www.redearslider.com/index_lighting_basking.html
If there isn't currently space in your configuration to allow for light fixtures, you need to think about how to rearrange things to make space. There are lots of good examples on here of lighting setups in the Indoor Habitat section on the main part of this site and also in photos in the photo section on the forum. If you want examples please check these out.
It's critical that turtles bask. If they don't, they'll get sick and have shell infections. They have to bask under uVB light in order to absorb the calcium they eat, which is essential for their shell and bones. Also, their shells must dry out thoroughly every day or fungus and bacteria will start to grow.