I finally worked it out.
I got my hands on MSF standard 3.0.2 64bits.
Compiled the LPSolvePlugin code (from http://www.peno.be/MSF/) in a 64bit machine using lpsolve55.dll for 64 bits (got it from http://sourceforge.net/projects/lpsolve/)
Copied the lpsolveplugin.dll and the lpsolve55.dll to the plugins folder of msf
added the following in the web.config of my webservice (can be added to the app.config of a desktop application if necesary)
<configSections>
<section name="MsfConfig"
type="Microsoft.SolverFoundation.Services.MsfConfigSection, Microsoft.Solver.Foundation, Version=3.0.2.10889, Culture=neutral, PublicKeyToken=31bf3856ad364e35"
allowLocation="true"
allowDefinition="Everywhere"
allowExeDefinition="MachineToApplication"
restartOnExternalChanges="true"
requirePermission="true" />
</configSections>
<MsfConfig>
<MsfPluginSolvers>
<MsfPluginSolver name="LpSolveLP"
capability="LP"
assembly="LpSolvePlugIn.dll"
solverclass="SolverFoundation.Plugin.LpSolve.LpSolveSolver"
directiveclass="SolverFoundation.Plugin.LpSolve.LpSolveDirective"
parameterclass="SolverFoundation.Plugin.LpSolve.LpSolveParams" />
<MsfPluginSolver name="LpSolveMIP"
capability="MILP"
assembly="LpSolvePlugIn.dll"
solverclass="SolverFoundation.Plugin.LpSolve.LpSolveSolver"
directiveclass="SolverFoundation.Plugin.LpSolve.LpSolveDirective"
parameterclass="SolverFoundation.Plugin.LpSolve.LpSolveParams" />
</MsfPluginSolvers>
</MsfConfig>
and it worked.
Hope it helps!