Right-click the file in the library. I presume you can use the Control key to select multiple ones at same time. However, I'm using LW 5.2 & I do not see any such options for my incomplete files. I did not think LW 5 had the option to continue incomplete files that refuse to load.
The only process I'm aware of is to revert back to LW 4, select the incomplete files in the Library & press Resume button. This will force them back into download window. Then you could close LW, upgrade back to LW 5. There is a sample image about how to do this here
Downgrading to LimeWire 4 from 5 & resuming incomplete downloads.