I have my site hosted with Powweb too. and got one of my sites running with no problem at all.
Just upload all the WB files in a directory in htdocs
Via Billing/plan set up a mySQL database
Run WB and set the correct information during install (database name, user name, password for the database, no need to set a prefix)
You should check also if safe_mode-off in php ini. When it's not, download the current php.ini from Powweb, change it accordingly (safe_mode=off) and upload it to the directory WB is in.
Here you can find the current php.ini:
http://space.powweb.com/phpinfo.phpYou should be up and running