well you can't really run the simulation before you have the keys from the walk designer right?  you can clothify the object, but it needs the movement provided by the walk designer to find the verts to collide with.
if you have a problem with other things after using the walk designer, i would recommend getting the walk you want and saving the file, then closing poser and opening the file.  that should clear the error there.  then go ahead and apply cloth dynamics.
note that the premium method for using dynamic cloth is to grow the cloth to fit your figure first, and then save out the object at the point at which it fits, then reimport the obj and save to the library.  i have occasionally had problems with poser having an issue looking for the obj when it wasn't saved to the library.
if you want the cloth to be tight, you can use a magnet to very slightly shrink the parts of the figure the cloth will be tight at and save this.  grow the cloth to fit this.  then start all your animations with 1 frame that is this shrunken part, the rest normal.  this will preserve some of the tension you need to hold the cloth in place.

go that way really fast.
if something gets in your way
turn